Even fast LACP needs three seconds and that's on the same collision domain.

How does BGP actually detect a link is down? Keep alive default is 30s but that can be changed. If you set it to say one second, is that wise? Once a link is down, that fact will propagate at the speed of BGP and other routing protocols. Recovery will need a similar propagation.

Depending on where the link is, a second can be a "life time" these days or not. It really depends on the environment what an appropriate heart beat interval might be.

Also, given that BGP is TCP based, it might have to interact with other lower level link detection protocols.

I have both my own multihomed ASN and operate my own nameservers. The latter has usually been about as fast for failover overall in practice. BGP may look to converge near instantly from your 2-3 peer outbound perspective but the inbound convergence from the 100k networks on the rest of the internet is much slower and has a long tail very akin to trying to set your DNS TTL to 0 and having the rest of the internet decide to do it slower for cache/churn reasons anyways.

The bigger problem, and where BGP multihoming is most handy, is it's just so much easier to get a holistic in+out failover where nothing really changes vs in DNS where it's more about getting the future inbound stuff to change where it goes. E.g. it's a pain to break an active session because the address had to change, even if DNS can update where the new service is quickly.