The actual answer beyond appealing character designs is that they're often relatively simple and designed around very long term progression. You get new units over time that you level and strengthen over time, and essentially the more units you have the more situations or better numbers you can potentially make. In a roundabout way, most gachas are effectively just an extrapolation of Cookie Clickers appeal, on a gameplay level. Of course individual games can have further draws beyond that (appealing designs, somewhat fun moment to moment gameplay), but this is the most common aspect among all the games in the genre.
