Some parents wait so long to potty train their kids because they are either too busy or don't believe that their kid is ready to potty train yet.
Some kids will eventually potty train on their own and some kids still need encouragement and training.
A kid still being in diapers full time and not potty trained is usually not a cause for concern until the child is 4 or older.
Kids under 4 are sometimes not ready to potty train and that is okay and if you actually try to potty train too early it can make the process of potty training take much longer and lead to more accidents.
I never potty train my kids until they are at least 3 or over as I've tried before and it always took longer.
My parents said I was potty trained at 18 months, but in reality I still peed and pooped my pants a lot until after 2 years old and so while I could use the toilet, I was having to still be taken to the toilet.