It is the Sheikhs club, If he wants to buy Messi for Man City, and loan him to NYCFC, at "brother in law rates" WTF does it have to do with the MLS?
Vice Versa also applies.
Well, it has to do with the entire MLS structure as it stands. Time Warner bought boatloads of superstars for the real Cosmos, and the whole league went under chasing them (not to mention the amount TW lost). MLS has built its base via grassroots efforts. It is slow going, but it it's proven viable in a way soccer has never been here. If CFG did it, it wouldn't be long before the arms race launched.
MLS and US Soccer are very much in bed together, and I think there's a very conscious effort to not simply be another premier league. Of course they would aspire to that sort of global recognition and the quality, but I think they are rightly looking at building out further development infrastructure before turning their sights fully on being a global league. There's so much quality that can be added via CONCACAF and earlier talent identification, I'd prefer to see us become a league full of great young talent supplemented by some experienced high level players that really turns a profit on selling players to the big leagues. If I were making a 20 year plan, that's what I'd go for. Honestly, it might only be 10 away. If they could sort out a way to sign some of these guys like Pulisic or Flores and then sell them on to Dortmund, the money would work itself out while maintaining some equilibrium.
I'm all over the map. In any event, the answer to your "Why?" is simply that it's not the way MLS works. The league is a cartel because that's how it's organized. It's not the most capitalistic structure ever, but you knew that. They are all protecting each other and keeping expenses down while maximizing revenue (to strengthen development I sincerely hope).
It's really a pretty interesting structure. As much as I'd love to see teams sign lots of big names, I do think there is a niche for MLS to be a pretty widely known and respected league due to its level playing field and the inability to solve problems by splashing cash. As many people as there are who constantly complain about the big clubs just spending everyone into also-ran status, it's feasible that MLS might gain some backlash fans in non-American markets.