Actually, for Greg's goals in RuneQuest, 20 broad skills isn't enough. But Runequest also isn't as combat focused as D&D.
Plus, the "20 skills" of D&D is fundamentally wrong (tho' technically correct by the letter of the rules); transport proficiencies and tool proficiencies work exactly the same way, and each are at least 10 more (out on pit stop during errands, so books not to hand). Wagon Proficiency is functionally a skill, just one without a tight attribute assignment to wisdom.
Not counting the weapon proficiencies, either.
Also, the difference in the numbers and types of skills makes a strong textural difference. You don't always succeed on casting in RQ, but you do in D&D, due to the casting checks.
This directly affects the fiction by causing uncertainty and the possibility of side effects, which, with critfails being standard in RQ, means a different approach to the fictional state by players familiar with the system. RQ combat is unrealisiticly deadly; but the other skills are more realistically narrowed to about one medieval guild per skill. Which makes them more interesting to engage with for mechanically minded players. (But deady to the fun of "Sommetimes we even roll the dice" types.
There is, however, a HUGE problem with large unbounded skill lists. They are just unworkable. I'll use a character sheet from my long ago 80's CoC campaign that I dug out as an example (I will also note, I was wrong, this is CoC 3rd Edition, still really old):
There are 72 skills on the character sheet, and the list is 'open' (there is some space to add more). Weapon skills are not listed either, you have to write those in (presumably there are potentially a vast array of them). The character in question seems to have 10 skills with a rating, so she will be baseline in all the others, meaning she has anywhere from a 0% rating up to 25% depending on the skill. Of the 10 she's got points in, 5 are at or below 30%, and 2 are at or above 60%. She's a 'parapsychologist', which IIRC is a specific occupation or at least a variation of 'academic', yet her psychology skill is only 35%. So, frankly, this character has few really usable skills, library use and read/write (English) are the best.
Now, suppose she has to conceal herself, what skill does she use? Well, there is Camouflage, with 25% (the base level, we are all able to sort of do this), BUT there is also Hide at 10% (again, base level). Which would apply? Some situations might seem to mandate Hide, but MANY could be either one, flip a coin! Why do we need Camouflage? Why are people so much naturally better at it than 'hiding'? There is also 'Sneak' which has a baseline of 10% as well, but if you are sneaky shouldn't hiding and sneaking be pretty much correlated? One is unlikely to achieve great ability in one and not the other for sure...
Likewise we have Debate, Oratory, and Fast Talk. Why do we need all of these? I mean, sure, I can kinda parse some difference between them, but MANY times you will simply want to 'talk' and now you have to pick from several skills, and you may have drastically different ability to employ these often subtle variations. Nor would I limit the world to these 3 realistically, there are certainly many other variations of human communication at that level of granularity! Nor is it clear why you would use these instead of other skills like Law, Psychology, Speak, etc. depending on the situation. Since CoC doesn't clearly distinguish whether intent or action is the relevant factor we can VERY OFTEN have many choices.
All of this, combined with the lack of any 'check valence' or 'pacing' makes using skills in CoC/BRP quite painful. It is really a very awkward system by modern standards and when put in contrast to something like PbtA I honestly cannot even contemplate using it anymore.
As for 5e, well, at least very little is made of proficiencies in 5e beyond weapon/armor really. Honestly I played through 2 campaigns and maybe used them a couple of times in each. 4e's design is also MUCH stronger, where each of the 19 skills is very broad and represents something like an 'approach' or 'knack' more than a hard skill. The binding to specific ability scores helps there too.