Tell that to the old animated theatrical shorts that were made back in the 1930's-1950's before the television.
Honestly, the reason why the tv cartoons in the 50's and 60's have such limited animation and limited style is because of budget.
Since theater revenue isn't a thing once the TV came into the picture and the TV grants full access to shows via broadcast, they have no choice but to cut back on budgets and rely on commercials for funds, especially the animation budget. Along with the perception of "animation = kiddie shows" that brewed upon in the late 50's.
UPA animation studio pioneered this style of limited animation and limited backgrounds. With other animation studios trying to mimic the animation style at that time.
But theatrical shorts back at the 30's-50's are well animated because of the higher budget and the revenue from the theatrical releases.