Quite obvious. Large lightning nodes which are the default hubs for custodial wallets have respectable part of the network's funds.
I see. But Blixt, Phoenix and Muun also have such default hubs for all their users. Nevertheless, routing is much worse for these wallets (comparing with LNBits, for example).
Not sure about Blixt and Muun, but to the best of my knowledge, Phoenix is (still) completely self-custodial. So while you do only have a channel with their node, making it a 'hub' of sorts, it is different to custodial wallets, where you don't even run your own node with your own channel(s) at all, but just have a balance on someone else's node.
This highly facilitates the job of the 'hub' provider to make sure every user has plenty of inbound and outbound capacity.
Picture this: in a Phoenix (also Breez) scenario, providers have to rebalance the channel that connects users to their hub,
for every single user, if needed.
In a custodial scenario, they have to do that
once for all users. Because it's 'just' one big node to manage, with liquidity going in and out. And transfers between users can be made without even touching Lightning. L3, if you wish.