Because … one day this deal will end and they won’t have anything to show for it that they don’t already have. Disney isn’t suddenly going to add millions of Spider-Man-only subs that they didn’t already have for their other Wonder stuff. The population that does not turn into mcu fans is very small.
They can take that FU money and throw it behind the X men series, F4, the other Marvel characters they already own and create a permanent library of content. Or he’ll throw that money on stuff that FX makes like Alien or Shogun.
Disney is all about the permanent library