Adoption in its own right (aside from any issues or neglect or abuse) is consider a "trauma".
If you consider your own children and imagine how they's cope at say 12 months (when my son was placed with me) with being taken away from everything they know - not just the people but the place, the food, the routine, the smells, the light, the amount of traffic. It's akin to an adult being abducted by aliens.
So adoption is a trauma. And many children also have to deal with the effects of neglect and abuse or institutionalisation in their early years when the brain is developing and it can have long term consequences in particular for their executive processing skills. (Which @Hels20 tend to start manifesting at around 6-8 years old.)
Poor executive processing skills can mean behaviour which is very difficult to manage and its a long term process to improve them using structure and repetition, organised sport and learning to play and instrument are known to help and yet they are also the kind of thing which a child with poor executive processing skills can stuggle with.
In our case, DS (now 13) has made massive strides with the support of school and CAHMS - and some real self awareness on his part. Tweaking his hobbies so that they are "calmer" ones, being structured all help.
No doubt we'll have another blip as he goes through his teenage years but fingers crossed we will get through that too as we did his pretty grim 7-11 yrs.
Plenty of birth children are equally challenging as @Italiangreyhound said. The difference is just the proportion of children who suffer issues and the degree to which they experince those issues.
Based on nothing scientific I have always reckoned that 20% of adopted children have significant behavioural problems, 20% have no behavioural problems and 60% have a degree of problems that vary over time.
You don't need to be sad for us, I love my son 100% exactly as he is and he loves me just the same - most of us sucessfully negotiate the grim years eventually with a few more grey hairs and a lot more wisdom. That's not to diminish the struggle at times.
It will all work out in the end, and if it hasn't worked out yet then it's not yet the end.