Why would they? They have more control of their own chain and they can select the BPs themselves and have almost the same security as EOS main chain and also have an ecosystem that is more aligned with Ono's specific application. Ono can also control the distribution, naturally. Lots of advantages. Same as Wax.
High value dapps might as well just run their own EOS chain with their own distribution. Low value dapps may skew more towards the main chain.
The same security if the select their own block producers? If they select their own block producers (and continue doing so) the governing mechanism of EOS is almost invalidated, and as a user I would certainly not trust them over a chain with well known block producers and a big eco-system.
I agree, that is the power of EOS mainchain. Any of these side-chains, if not using the EOS mainchain BP's is not "decentralized.