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human
ELI5: In weightlifting, why is it, that the relation weight/repetitions is not even close to proportional?

So I have been doing semi-competitive weightlifting since I was 16, but it just now occured to me, that the relation between the weight on the bar and the amount of reps, that one can do with that weight is neither linear nor proportional.
Say I can do 440lb for 2 repetitions, that means I will probably be able to do 470lb for 1 rep at best, which is not even close to 880lb (440*2). 

Furthermore, if someone benches 225lb for 1 and I can bench 450lb for 1 I am factually twice as strong as that person. However, if a person benches 225lb for 5 repetitions and I could bench 225lb for 15 reps I wouldn't even be close to being twice as strong. 
Why is this? Given that physically W = m*g*h, with both g and h not changing, I need the same amount of force to lift a weight twice than I would to lift a weight that is twice as heavy once.
gpt
You're confusing "force" (N, Newtons) and work/"energy" (J, Joules).  The force required to bench 225 for one rep is the same as the force required to bench 225 for a hundred reps: the weight is exerting 225lbf=1001N on you so you need to push with slightly more than 1001N to start it moving up on the concentric part of the lift, and then slightly less than 1001N to gently bring it back down on the eccentric part of the lift.  It doesn't matter if you repeat this process for endless reps, the forces required are the same for each rep.

In your example above, if the max force you can exert is 470lbf, it doesn't matter how many reps you can do at that weight... The 880lb bar will be pushing down with 880lbf and you'll be pushing up with 470lbf and nothing will move.  Even if you could magically summon up 881lbf, you would still probably fail the lift because human muscles get tired quickly and peak force production quickly drops.  That's why bar speed is important.

Energy does scale linearly as you suggest: two reps moves the weight through twice the distance of one rep, so the number of calories burned is double, but this isn't really a concern for strength lifts.  

(Btw, you used the formula for "work done on an object", which is technically always 0, since the weight ends up in the same position it started from.  But that's not what we're interested in here.)