I've had this same problem for a long time. Just recently I've gotten super into playing, and am actually gonna load it up after finishing this comment, lol.

I made a character and wound up caring a lot for them, which is new. I made a genderswapping alien (alien form is feminine, disguise is masculine), and I'm nonbinary, so I'm rather attached, lol. I also committed myself to trying to build a proper house for them, more than a box with a roof, and that's been pretty fun, learning the building tools and finding new things to add to the home--which is decorated in a way that is not my taste at all, but still looks pretty nice, so that's an added challenge. They just got married, and I made a point to take my time with their spouse. Went on lots of dates, no woohooing or moving in together, so I got to care about the spouse and their relationship as well in that time. They're about to start looking into kids. The farthest I've made it in TS4 with a kid is toddler age, so I'm excited to see what else is in store there. This is also my first time playing with an alien.

Honestly I just hit the right vibe with this Sim, and am using them to try completely new things and see what all the game has to offer that I might not look for in a normal playthrough.