I read yesterday that all computers issued for Congressional staffers after 2017 have full disk encryption enabled by default.
Do you hear that? That's the sound of me face-palming.
That's an illogical assumption to make. Not encrypted at time of issue != not encrypted ever. Who says all previously-issued laptops weren't encrypted at a later time?
It could also mean a change in policy which makes official what was already happening.
Did you see the picture of Pelosi's desktop monitor? Pelosi was on the House floor at the time, wasn't she? Why wasn't there even a screen lock?
Any modern system(even on the low end) that you could conceivably want to trust important data to can handle encryption requirements with ease.
One problem if we are talking about Bitlocker specifically is that if the drive reports that it supports encryption, then Bitlocker offloads the responsibility to the drive. And the drive encryption might be badly implemented.
Edit: Also, stealing a laptop that belongs to the office of the US Speaker of the House will essentially never end well. There are endless examples to suggest that yesterday's fracas wasn't thought-through by the participants. This is one of them.