Finn was raised in that apartment complex and Id assume Margret at least would have attempted to send him to a school. There are weird colleges and schools around Ooo.
Also PB actually liked Finn enough to take time from her duties to hang. Filming movies and coming around for movie nights. Both have a strong sense of justice and depending on how long Finn and Jake have been around, they got to be familiarized around one another. Both also have a strong sense of trust in the other. Finn trusts PB to be indiscriminate in handing out justice and honest in her questgiving and PB trusts Finn to carry out her assignments with the upmost respect.
Of course with PB's regal and nice nature, eventually Finn would grow to have feelings for her. She could've squashed it right then and there like Marceline did, but decided against it. I personally think she was waiting to snatch him up once he hit a suitable age and viewed the world in a different light. Otherwise she wouldn't be goading her to kiss her ass in Richardio or not focusing on becoming 18 again in Too Young and blowing Finn off.
This is all of course the first 2 or so seasons before shit got all deep.>>109898416
Yet shes better in many ways. I just like their pre-lorefeels sides better. Not all over the place and easy to digest while still being great entertainment.
I'm probably acting like a sperg here, but there is more than meets the eye than "unf prubbles I just wanna fuck you pls." I REALLY god damn wish they butted heads more in the middle to later seasons to solidify Finn's "maturity" arcs. As much wasted potential as Marceline and Finn having more co-op adventures.