Because one team needs to get the US-1 CCL slot. And, like, the stars and stuff. Also, can you imagine how mickey mouse and oranges slices the League would look if there were 3 completely different Real Champions. Also, if you have a regular season, that qualifies teams to the playoffs, and then after the playoffs you hand the winner a trophy, no one is going to give a damn about the Shield champion just because people say we're supposed to.
I like the Shield, MLS Cup and US Cup as three titles. It isn't unlike the League Cup, FA and league. Three different skill sets for three different competitions.
The CCL draw has a pot of 4 Liga MX clubs and 4 American MLS clubs. Starting with 2015-2016, all the American MLS clubs got treated the same.
For instance, the US Open Cup involves downhill skiing! One skill set for three competitions. And at the end, there are three clubs with trophies, and one of them is the Champion.
I like the downhill skiing idea. I guess I'm at a point where I don't give a crap about champions, I just like watching the games.
So what you're saying is you're a soccer fan, not a competition format fan. How adult of you. (And the World Cup downhill is easily one of the most entertaining sporting competitions on the planet.)