Several robo trading services allow investing without a fee up to a certain amount. Why would someone not take advantage of this and spread their investments over several such services to minimize fees? Is the only reason convenience?

Several robo trading services allow investing without a fee up to a certain amount. Why would someone not take advantage of this and spread their investments over several such services to minimize fees? Is the only reason convenience?

Looking at services like Betterment, WealthFront, Charles Schwab, Motif. Some of these said under a certain amount was managed free.