Apple have a TV channel and they bought the rights of something popular to show on it. The strategy is very simple.
But they didn't buy an MLS club or an MLB team, they only bought the TV rights. What we've seen so far suggests they don't want to be involved in sports development, but rather sports broadcasting. Buying United doesn't get you the rights to show their games live, and probably never will. You'd hardly call the ability so show 90 minutes matches the day after the game was played an enticing prospect for subscribers. And even if they thought it was, just strike a deal with United to show their highlights. You don't need the club for that.
Worse still, it could be a future problem owning a club. What if the PL TV rights come on the market again and Apple decide they want them? Owning a club could be a conflict of interest that might stop them bidding for the TV rights in the future. If showing more European football is their aim - which seems entirely likely - then being a club owner might actually muddy the waters.