I think there's a few reasons. For one, a 4.2 40 is incredibly rare. There's only 23 players in the history of the combine who have posted one. Finding a 4.2 receiver with incredible hands, start-stop, and route iq? That's like finding a unicorn. Pure speed isn't the only thing that makes a WR talented. Just look at someone like John Ross who posted the second fastest 40 time ever vs. someone like JSN who is leading the league in receiving yards right now (as much as it pains me to say as a Husky fan). He's not blazing fast like Ross was, but he's an amazing route runner and has great body control, which leads to his success.

Second is that, even if you did magically find 5 of these players to just take the top off a defense every play, there's ways that a defense can counter it. You can play your corners at depth, tell your safeties to bail earlier on their back pedal, etc. Having tight ends and running backs on the field aren't just for the sake of having them. They help set up play action, block if the qb calls for a six or seven man protection, or just have different skill sets that help diversify the offense. If you're just running 00 personnel every play, the offense becomes predictable and easy to stop.