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: 
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.
COVID briefings are aimed at the general public (and many Deaf people have poor English skills due to a history of bad Deaf education) and are low on jargon (hence fairly straightforward to interpret). Interpreting into the local sign language is the right choice for maximum accessibility.

For a high-jargon scientific speech to an audience of scientists, transcription may be the better option. You would need a specialist interpreter to manage such a speech; and it would be tricky for the Deaf audience member to view both the interpreter and the slides at the same time.