The best way of explaining I can think of is to look at historical armour. A knight in full plate can basicly walk through a storm of arrows and not really care (unless he gets really unlucky). That same storm of arrows can kill hundreds of unarmoured peasants. It isn't until you bring out the extremely heavy warbows with specialised armour piecing arrows that the knight has to start worrying. Point being, anything capable of slaying that knight can slay hundreds of peasants.
The problem is that once that knight gets into melee, that stops being true. Also, specialized anti-armour arrows/bolts or guns will deal with a knight nearly as effectively as they'll deal with a peasant at short range. In melee, 5 peasants with pointy sticks will probably take down an armoured foot knight; fighting multiple foes is, in reality, an extremely difficult proposition, even if they aren't well trained. The knight might be able to kill a couple, but then he's on the ground, held down, and having his armour torn off or getting stabbed in the face. That just doesn't translate very well to a wargaming system like AoS (Though the Middle Earth SBG doesn't have such a problem, and other skirmish systems tend to be similarly better at dealing with such situations in a bit of a more "realistic" fashion). 10 knights will fair relatively better against 50 peasants than 1 against 5, but eventually, unless they can prevent themselves from getting surrounded (ex: by holding in a doorway; a situation where hordes are drastically worse as is in AoS), they'll fall. All it takes is getting split from the other 9 knights for a few seconds and they're done for.
Most missiles in AoS can be expected to have a reasonable shot of wounding. The ones that wouldn't, like goblin bows, already have poor hit/wound values and no rend.
The value of a variable "toughness" is effectively already covered by save and wound values and rend adds a variable stat during the wounding process.
There's also the problem that being effectively unable to wound things with some units due to too much variance in hit/wound/save values creating very binary rock-paper-scissors gameplay, which is bad. You don't want games to be decided before the armies are even put on the table because of what particular units were brought.
That was one of the biggest problems in both WHFB and 40k. It's much less of a problem in AoS than it has ever been in those two systems (It's still a problem, and was especially notable with pre-nerf petrifex, and other factions with access to very high re-rollable saves. In WHF with weapon skill, strength, and save values all being variable, it was possible to have units that the opponent just could not reasonably kill with anything their faction has access to. This was most notably a problem with the WoC Nurgle daemon prince and Tzeenth Chaos lord, 1+ armor save cavalry, etc... Cavalry having +1/+2 to save over infantry, instead of an extra wound, meant a much more binary experience - either the opponent had the strength/AP/magic to deal with it, and deleted your units, or they didn't, and couldn't kill your units; monsters having 6+ toughness with 5-6 wounds had the same problem. Opponent has a cannon? You wasted 200-400 points on that monster.
The reality is, enough clanrats should be able to drag anything down. It doesn't matter how tough, big, or well armoured you are when you're mobbed by 100 (or maybe 400) 4ft tall rats with weapons. A monster can't be aware of everything around it and isn't infinitely strong. A knight can't see behind him, and can't defend against everything. Some of those rats will land some solid hits, or grab onto that monster/knight to make it easier for their fellows to wound it.
Usually, when making a compromise between "realism" - which is often overstated, and way too complex for most TT systems, and gameplay, it's best to err on the side of better/more fluid gameplay, which is what AoS's system has done with regards to the wounding process. There's an argument to be made that missile units should do a bit less damage with shooting and be a bit better in melee as a general rule, and that damage output in general is too high/it's too difficult to make units flee, but that's a different discussion.