So in most cases, a pure number based skill system doesn't make sense as the system doesn't allow for dice rolls. Tons of skills require pure number system, which in turn require dice roll checks.
I believe it can with mods, but the problem (as I see it), is that as for the base retail game... not every player the want to sell to understands (or prefers) the notion of a character's personal ability... Some consider the PC to be themselves in the world, and the concept of dice {specifically a failed dice roll}, is seen as oddly arbitrary and even resented by some when it happens. How many times have you read posts exclaiming "I shot him in the head X number of times and he didn't die!" ~when we both know that the weapon aiming [in the Fallout series] is controlled by the character's ability to use use the weapon; and the bullet's variable damage (in FO1&2) reflects a further layer of accuracy (and differentiates a headshot to the face, from one to the ear cartilage ~for instance). Even in FO3 the player's aim is actually attenuated by the characters skill in guns, but they assume its wherever they point; In actuality player aiming (with the mouse) is just an overly complicated way of selecting a target for the PC to shoot at.
**Personally, this awkward artifact of FPP/RPG is why I'd prefer it to be TPP at about 20' distant or more, because those minor details (that nag at the player's common sense) don't show up at that distance ~You can't see that the PC shot precisely to their face (and they didn't die). A low roll (if the new system had worked like that) would have just meant they didn't quite hit it dead center.
Yes I guess it's constant. In most cases it makes sense. In others where it doesn't, it greatly complicates the underlying coded system and makes difficult for player to control. In dice RPG, you just state your intentions to the GM, and that's that. Like, "I'm gonna hit him with my axe but not cause bleeding". You can't do that here, and what would be the point? Or jumping skill. In dice game, I can attempt to jump exactly 3.15m, and dice roll based on my skill determine how close I get to that. Here I would likely get a perk that allows me to jump twice as long as normal, hooked up with "air control" to make it more player centric but chance is completely lost.
The real benefit of Numeric and Perks systems is to easily depict the intangible and/or unusual character traits. In Fallout most perks were used to bend the rules a bit. In Fallout 3 perks were [IMO] misunderstood to be abilities; and this lead (I think) to them finally just becoming the 'skills' in their next game.