It's not the attacks that are the problem really. Pool attacks are ridiculously priced end-wise for example.
It's that the secondaries are needlessly expensive.
If you have, say, a corruptor or controller pushing their secondaries to keep a team alive while barely using their primary or not using their primary at all (yeah yeah, I know, bear with me) then why should masterminds be expected to do the same thing plus keep all their pets alive with their secondaries on top of it only that they have to spend way more endurance to do so? Now one could say the pets are dealing damage and using debuffs and controls and heals, etc... but as I said, even if it was even endurance the mastermind will be spending even more end to keep all their additional targets alive. Typically in my experience corruptors and controllers don't bother trying to keep track of mastermind pet health.... and that's on top of the pets being lower level than you are and die easier than any other player even if they were at the same level. That's not even taking into consideration how low damage say a mercenary mastermind brings to the table compared to most classes...