Because all of the named fighters were unnamed and the headliners aren't ALL that people buy the PPV to see.
Also, because they fucking can. The better question is why do you think everyone but the fighters needs to take so much of the profit generated by the sales.
Umm is this supposed to be a trick question? My guess would be because fighters sign a contract to be paid a specific dollar amount to work for the UFC, not a profit share.
If fighters or anyone wants a share of UFC profits, buy TKO shares, that is how the real world works.