I’m pretty sure gold is there because of Starwood.

Pre merger Marriott had a 15 night level (Silver), 50 night level (Gold), and 75 night level (Platinum).

Pre merger Starwood had a 25 night level (Gold), 50 night level (Platinum), and additional bonuses at 75 and 100 nights. (Made more complicated because you could qualify on stays too)

In both programs, most desirable benefits like breakfast and upgrades didn’t happen until 50 nights. The new combined program that we have now basically took the Starwood tier names and added Marriott silver to the bottom rung.