Essay incoming!
As you say, nobody will be able to provide a definitive answer as such but here are some of the common theories for why Year 1 classes appear to be becoming more difficult in general, as a national trend, year upon year:-
Covid babies
Some people see this as the sole reason for the decline. Other people see it as a complete excuse. The reality is most likely somewhere in between. Services were obviously cut dramatically during this time, which must have had some effect somewhere down the line (health visitor checks either not happening, being delayed or only occuring over Zoom rather than in person). A lot of early developmental issues slipped through the net. Depending on the individual circumstances of each family, early attachment and socialisation skills may have been affected (whereas, for others, the opposite might be true).
Life-style changes
Life is significantly different for most young families these days. Most families have either two working parents or a single working parent (with many having to work full time). Children are therefore spending less time with their parents than ever before (again, affecting attachment). Lots of infants spend long hours in private nurseries from a very young age, long before they start school. Many children spend long hours in wrap around care once they are at school. Family time is precious, and some parents may understandably feel less willing to discipline their children as they spend such little time together as it is. They wish their time together to be enjoyable, so children these days do genuinely seem more use to getting their own way than ever before, and many never hear the word no at home. Starting school, where there are firm rules and boundaries, can be a real shock to the system for these children.
Rise in technology
Linked to life-style changes, children today are far more likely to be babysat by screens for long periods (especially if parents are juggling working from home, rather than using wrap around care). You only have to visit a supermarket or restaurant to see how many infants are glued to a screen, even when they're out of their home. Research into the long term effects of prolonged screen time on young brains is still in the early stages, but the new advice on screen time for under 5s has been a long time coming (and will sadly be ignored by many). Ask anyone who works with young children, the link between excessive screen time and concentration is obvious.
Lack of SEN funding
The current state of SEN provision in this country is appalling. Far more children than ever are being diagnosed with SEN, or have suspected SEN which they may or may not be on the pathway for a diagnosis for (which can take years). Yet there are less and less special school places available. We are seeing children with some pretty extreme SEN needs entering mainstream education these days, far more than ever before. The lack of funding for these children is having a massive impact on all the children. It's not uncommon these days for Year 1 classes to only have additional adults as named 1-1's, rather than general TAs. This impacts everyone and is nothing short of a national disgrace. The government are not only massively failing children with SEN, but they're also failing their mainstream peers who are impacted by sharing a classroom with children who sadly are not suited to mainstream education. Young children often display copycat behaviours. For every class who display empathy to their peer with severe SEN needs, you'll have another class who are off the wall because they can't cope with the noise and disruption these peers may bring with them, through no fault of their own.
Poorer Communication Skills
Early communication skills are generally declining, year upon year (probably due to a mixture of everything above). In general, children are spoken too far less at home than ever before, and the percentage of children who are read to at home seems to fall year upon year too. It's a bleak picture which is beginning to impact massively in the classroom. Children who struggle to communicate their needs and feelings generally become more frustrated, which can sometimes result in aggressive behaviour. Children who struggle to listen and focus generally become less engaged in the work, which can also lead to 'silly' behavioural choices. It's a vicious cycle.
Educational changes
Sadly Year 1 in the 2020s resembles nothing like the Year 1 we remember from our own school days. The expectations the government place on 5/6 year olds in terms of attainment and curriculum content is ridiculous these days. Some schools do still run some continuous provision in Year 1 (role play, construction, outdoor learning etc) but, for many Year 1 classes, whole class teaching where every child is sat at a table doing maths or writing (for example) together is the norm. With less general class TAs than ever, this puts a massive strain on the teacher who is desperately trying to support them all. Some children are not developmentally ready for this. They miss the freedom of Reception and some do sadly (but understandably) rebel with a dip in behaviour.
I've probably missed some things, but these are the main 'theories' talked about often within education circles. It's a desperately sad picture which just seems to be getting worse and worse each year. I don't think it's down to one single thing, but all of the above definitely plays a part. I don't see Covid as a cop out excuse, it undeniably had an impact, but I think people are naive if they can't see that some of these problems were starting long before 2020. I last taught Year 1 in 2018 (I've been in Reception ever since) and they were a challenging bunch, even then. My Reception classes seem to get more challenging with more extreme needs each year. The picture continues to get worse and worse. It can't just be 'because of Covid'.