Idk, Kat has never been around to do more than fill in the blanks with speculation, but she'd have good reason to work.
Philburt's a couple bans away from having literally no income and, with his online history, no job prospects. Her having ongoing employment and some kind of an income stream, even if it isn't much, is a very smart idea, and being as she's been kicked around from place to place for most of her life I'm guessing she knows the basics of how to take care of herself.
Perhaps, though it begs the question of why she doesn't seem to feel any attachment to the person providing that comforting feeling. Say what you will about Leanna, she obviously liked Phil up until she didn't.
Leanna was a literal child when she met Phil though, and she seemed like kind of a weirdo so who knows how much experience she had before him.
As for why Kat doesn't care much about Phil, possibly because she
has had a very tough life: phil's some clueless momma's boy who couldn't reason his way out of a wet paper bag, he's got noodly little arms and can't even deliver in bed, plus back when Kat and Phil first got involved (I'm sure Kat is at deathfat levels in the current day) she could've probably beat his ass. While he was crying about wolverine figures to his doting mother, Kat and her sisters were getting locked in the closet by a screeching physically abusive nutcase and confiscated by CPS.
Plus just in terms of personality Phil is very touchy about intimacy overall, so I doubt they ever have any kind of conversation that goes beyond mind numbing small talk, it's all just "Yes, hello fellow mature adult, let's talk about some of the things mature adults speak about with their mature adult spouses! Being mature adults sure is swell isn't it? Ha ha!".
Kat is more of a man than Phil is in many ways. I'm sure she's fine with him as a roommate, but I doubt he does much to engender attraction or affection.