From what I understand, it think it’s because of zaps. You need a NIP-57 enabled wallet for that feature and it seems to be available on custodial wallets at the moment
Discussion
It's not only zaps that favor custodial wallets, It's also LN Address that's not solved for non-custodial mobile ln.
However my question was about closed source vs open source wallets.
Are folks not aware what's what or have we stopped caring?