I trained both of mine quite late (according to most on this post), I admit it was an element of 'laziness' with my eldest, alongside most things I read saying that it's best to wait for the 'signs they are ready' and 'boys train later than girls on average', so I wasn't prepared to start training before he was 2, then I fell pregnant with his little brother shortly after he was 2 and was in no way willing (nor capable) of trying to potty train alongside morning sickness for 20 weeks and then squat down with my big pregnant belly after that. Then I waited maybe 6 months after baby was born so that I had recovered and so that I was able to easily put baby down somewhere to deal with taking his older brother to the loo/clean up messes. Eldest was potty trained within a reasonable amount or time, I can't remember exactly but maybe a few weeks or so, I started and didn't stop with him until he was trained (so somewhere around 3.6/3.9) and he was night dry by himself by 4 years old. However, I did buy 'biodegradable' nappies for him to help reduce the landfill issue (around 60% biodegradable iirc) and I went straight to toilet with him, I could not be asked with faffing around with smelly potties or having to keep emptying out and cleaning one. He got chocolate jazzles as rewards to potty train and I used training pants, he hated feeling wet/dirty.
It went so well with him I assumed it would be the same with his little brother, maybe even a little earlier with him with having his older brother as an extra toilet using role model... I was wrong... I tried training his younger brother several times after his 3rd birthday (summer born so perfect time of year to go bottomless or in just pants), I tried chocolate button rewards, I tried the training pants, I tried explaining to him that if he goes on the toilet he won't need to be cleaned so much because he absolutely hates being cleaned up down there (screams and cries even at preschool), he is at preschool (nursery) so has other children to model too, I gave it a good shot several times since his birthday but he just didn't care if he was wet or dirty, I tried just straight going to pants and he still wee'd and poo'd in them and just kept playing! He would never tell me he needed the toilet but he would go and hide to do a poo. I gave him breaks of going back to nappies because we were both becoming stressed, he seemed scared of doing a poo on the toilet. He and his brother just had a week off school last week so I tackled it once more, but this time I bought loads of 2nd hand toy cars, some small pots to put small treats in and some toy animals and wrapped them all up and put them in a basket for them both to take a surprise every time they used the toilet successfully (so as not to leave his older brother out, I read this idea from someone on MN so thank you whoever that was!), it worked! He loves cars so finally the reward was worth the effort for him. He is night dry already (from the day I started training again 2 weeks ago, I suspected he would be an odd one and night train before he day trained as he has had concurrent dry nights on occasion before), he is having maybe 1 accident a day with a wee or a poo where he doesn't make it to the toilet in time, but he does actually go and finish on the toilet now, so he is getting there at least. He is dry when we go for outings as long as I take him to the toilet before we go (he can hold his wee for hours on end!), I do wonder if it would have been different if I hadn't followed the current mainstream advice of waiting for 'readiness' and had started around 2 instead, or if it still wouldn't have gone smoothly because he just didn't care about being wet or dirty (my husband found him one day last week hiding in the playroom where he'd done a big wee and then lay down in the puddle to try and hide it... he was completely soaked...
However, they are both going to be trained prior to starting school, even by UK standards, so even if I started late I still did it 'in time' in that respect (nursery didn't mind him being in nappies, his class is 2-4yo so many of the other children are still in nappies). He will be 3yrs and 9mos in a few days and I would say he is now basically (finally) toilet trained.
TLDR: I would blame current mainstream advice of waiting until they are ready and being 3 and in nappies not being considered an issue as well as an element of laziness/needing timing to be right too. I would still expect more NT to be trained by age 4 however.
Sorry for the long post.