In order to put the epoxy floor down at this point, they're going to have to get every bit of the paint off the concrete first. That means either sandblasting or basically grinding it off. Both are expensive and time consuming. There's a contractor who owes me for some bookkeeping who I've talked to about doing this. We had to get the doorway watertight first because they also didn't slope the garage toward the door like they were supposed to. Instead, they left it flat.....ish. There's a dip in the middle of it. They also didn't slope the first section of concrete away from the garage door like they were supposed to, so water was coming straight in when we got a hard rain, and we ended up with a big puddle in the middle of the garage every time.
My boyfriend and I met when he was in the middle of having this house built in 2006. I've been around both residential and commercial construction since 1991, so I watched this house being built as I either bit my tongue or watched as the things I pointed out to him were swept under the rug by the contractor as either no big deal or I didn't know what I was talking about. The garage floor being slippery when wet after they painted it was one of the things that I pointed out, that the contractor blew off when he brought it up to him. If they'd mixed a medium grit sand with the paint before they put it down, it would have had grip to begin with. That's an old trick for a temporary fix until you can afford a permanent one.
Since the contractor who owes me is roughly 30 mi from here, and the construction industry is still struggling so in his area, him having the money for the materials and machine he'd have to rent to grind the paint off just hasn't happened yet.