you have 2 master resellers who then have 10 resellers each who then have 20 shared accounts each, so from your 2 clients you have 400 accounts on your server so if you do not limit your master resellers just a few can overrun your server and you have no direct contact with whoever then place on your servers
I find it, if not cheaper, certainly "less trouble" to sell someone a VPS with a Control Panel than to give them a Master Reseller.
The main thing as a hoster is to be profitable, you are not doing anyone any favours by being too cheap, as that will only end badly for you and yours customers.
Therefore you have to be very conservative with space to give away to Master Resellers, and it certainly cannot be "Unlimited".
We like to give "more than enough" space to our users but not enough to encourage bad behavior!