That's because Tholians originally came from ST:TOS, and TOS had a relatively large number of non-Sapien races. TNG made bumpy-foreheads a standard, and later retconned it into the universe with the Progenitor storyline - probably as a cost-saving measure - but TOS was full of truly alien races which looked nothing like humans. Most were one-off encounters, and the recurring aliens tended to be humanoid: Klingons, Romulans, Vulcans. Cost, a recognition that viewers were going to struggle with identifying with non-humanoid body plans, and probably realizing that fleshing out a truly alien psychology was a lot of hard work; easier when the Enterprise has only a brief, single-episode encounter with a race.

TNG, and later series, went hard-core on the bumpy foreheads, and most of the few non-humanoid aliens were some sort of nebula or energy creature - far cheaper to render. At one point I hoped that with how much of new series were CGI, and his cheap it had become, that new series would ditch the Proginator-dominated interactions and re-introduced alien aliens. Lower Decks did, a bit, and The Orville (ST-adjacent) did a little better than the usual live-action ST. Bab5 was still mostly humanoid, although two major, recurring species were very inhuman.

Anyway, TOS did a commendable job of populating the universe with really alien aliens, and did so long before CGI. It's one reason why I think TOS is still the best ST.