Question: Watching various COVID-related briefings, I notice there is always a sign language interpreter. This is clearly necessary, yet thinking about all the conferences I've attended in the past, literally none of them have had a sign language interpreter.

How do conferences work for deaf scientists? Do deaf scientists simply not attend conferences? If so, how does one even work in fields such as computer science where conferences are an integral part of the academic experience?

Answer: 
In what was a very large astronomy conference I have seen a sign language interpreter (actually they had two, who round swap every few minutes during the talk) in certain sessions (presumably going to the sessions which the deaf scientist(s) was attending). I haven't seen this at other conferences, but whether that is because of their smaller sizes or because deaf scientists didn't attend I don't know.
They do not work. Most conferences in most fields of science do not work well for people with any sensory/communication disabilities.

A few online conferences offer automatically generated captions. These help some but they are not very accurate.

Edit: The fact that conferences are ableist is not because they are intentionally ableist, but rather because of ignorance. Many conference organizers would be willing to adjust their conferences if they knew how to do so and had the funds.