Because its at a completely different pace and crazy high level where the smallest margins count, where players have to play to the top of their ability. Not all can develop the same skills- if you could then most players would end up at the same ability levels.
When you really think about it suggesting all top level/pro players should be able to play with both feet is nonsense. they probably can if you slow it right the way down or step down in quality levels.
Its like saying - why can't he beat the first man with his cross, why can't he convert that chance etc? Why did he miss that tackle - they train at it each day!
it doesn't take into account the level, quality and speed, pressure, that there are opposition players training their entire careers too to deny the time and space and defend against it.
Comparing it to yourself is just daft. I used to train my left foot, thought I was decent with it. But put me up against someone at the same level as me or better and I'm going to naturally stick with my strongest foot as the left just wasn't good enough and I could probably do nothing more than use it to bounce a ball off in a dribble occasionally.
There's always mad assumptions that because a player isn't good at something they don't work at those things. Yet we live in an age where the players are measured and coached to within an inch of their lives.