Can't US Senators cast absentee votes (through giving temporary power to a colleague for instance) ? Or is it just because the self quarantines suddenly happened today and they didn't have time to do the paperwork ?
No, we don't have a modern democracy
Isn't it common in some State legislatures (up to having sticks and canes to push the voting buttons) ? But maybe it's more accepted usage than codified... Seems incredibly explosive in any sort of crisis but I guess you could make the argument that you have a lot more chronic absenteism in institutions that have this.
The basic answer is that legislatures can write their own rules. The Senate has no rule allowing for it, those state legislatures do. (Or vice versa.)
Tradition in the Senate has been pairing (a vote for/against "pair" and say the "for" cannot make it, the against also does not vote), because it's been rare for Senators to be significantly but temporarily absent due to the way Congresses were scheduled. Only once Congress became a year round session essentially has it been a thing, they have in the past brought Senators from their literal death and sick beds to vote.
There may be a constitutional question here too, because Senators are supposed to represent the State. So if I'm the Senator from Nevada, and I give proxy to the Senator from Alaska, how can he truly represent Nevada? In states with split delegations the Democrat would not want to give his proxy to a Republican. So on and so on.
I don't see why a Senator could not wear a clean room type jumpsuit in to vote.
