If you look at the numbers - the highest number of goals scored off set pieces is 14 - and that was for Stoke - which if you look at total # of goals scored - was just 36... it's clear they were missing a significant component of their game outside of set pieces. Set pieces alone do not win games or seasons. I'm guessing that statistically, there's a fairly narrow range of avg # of set pieces that occur in a game. So you're only going to get so many chances - the teams and players that can capitalize on both are the ones that score the most goals. Look at Man City and Man United- they've "maximized" their set pieces and also show a very strong game in regular play with 93 & 89 total goals. If Newcastle could capitalize on more of their set pieces, they could bring it up to maybe 66 total goals. Whereas Blackburn and Chelsea need to score more from the field...
Thinking we're going to just have all tall players so we can win all the set pieces is just plain dumb. The height certainly can play a role, but the reality, you need the speed, agility, and skills to be able to get yourselves into scoring positions and create opportunities. Shorter players have an advantage because of their lower center of gravity and short legs - making them much more nimble - in a football analogy, running backs (with some exceptions) tend to be much shorter so they can make frequent cuts vs. wide receivers who are generally taller so they can run faster, stretch the field, and reach up to grab the passes. WR's just need to be able to make 1-2 cuts per play. Think a giraffe vs a cheetah.