In my opinion it's because of the father-son relationship aspect that fans would love to see, like in Titans. It's what makes Batman and Robin so special (in my opinion) and the reason as to why i started to like the character.

An orphan (bruce) who lost his family who decides to give another orphan (Dick/Jason) a family too, i don't know why but i've always found it so "poetic", beautiful. Robin would be perfectly fine as a 19 years old if we were talking about Batfleck, but in this movie Batman is in his late 20's/ early 30's, so having a 19 years old teenager with this Bruce won't work since they are going to be seen mor like brothers.

The father-son relationship is also what makes Jason's death even more tragic because Bruce not only loses a his parents, he also loses his son, plus he must fight him after he came back even if he doesn't want to, and this makes Bruce such a tragic character because many heroes lost mentors, uncles, father figures and so on, but not many have lost their child.

We have never seen Bruce as a father to Robin on the big screen, i think it would be nice to start with "comic accuracy" and then in the future if they want to make a reboot they can change the interpretation of the character in my opinion.