On January 20th, Avril Haines was Biden's first cabinet confirmation, receiving an 84-10 vote in the Senate. Similarly, Biden's nominee for Defence Secretary was confirmed in a 93-2 vote. Why is it common for not every senator to cast a vote on such issues? I have searched but haven't found a reason why (I'd like to clarify that I am confident their reason for voting was not self-isolation reasons- that would be understandable). Is it because they are busy with constituency issues in their home state? Or other political reasons that they do not want to cast a yea or nay vote?
Because nobody cares about their vote.
The Senate is divided 50-50, but the numbers you cite are nowhere near that. This indicates that most Republicans were voting in favor of confirmation. So these were bipartisan votes where basically everyone (aside from a small number of dissenters) agreed with the confirmation.
Let's set aside the dissenters for now, because you didn't ask about them.
Bipartisan votes are hard to campaign on. For legislation, a senator can tout the fact that they sponsored or co-sponsored a popular bill, and/or talk about amendments and other committee actions, but with a confirmation, all they can really say is "I voted the same way as everyone else." This does not make good campaigning material, and so there is little incentive for the senator to actually show up and vote.
Senators have a number of other things they may consider more important than voting for a bipartisan confirmation:
- Fundraising and campaigning (not really relevant right after an election, but still important in the general case).
- Conferring with each other, aides, lobbyists, and the executive branch generally.
- Drafting new legislation.
- Making speeches and doing other forms of public outreach.
- Reading and interpreting opinion polls.
- Planning and coordinating all of these activities.
On the other hand, if a senator makes a habit of skipping many "unimportant" votes, then it may be used against them in a future campaign, so they need to balance the value of their time against the long-term cost of skipping votes. It's also a bad look if too many senators skip the same vote at the same time, so the leaders of both parties generally try to minimize this, with varying levels of success.
The dissenters, on the other hand, very much want to show up to these votes, because it enables them to say "I voted against the corrupt political establishment in Washington" or whatever brand of anti-establishment campaigning best fits their politics. At the same time, because these votes are basically guaranteed to pass, the dissenters need not worry about the consequences of a confirmation failing (which might otherwise be seen as divisive or even dangerous, depending on the position being filled).
It's a combination of things. They can honestly tell their constituents "I didn't vote for that cabinet member you hate so much" or "I didn't vote against that cabinet member you love so much". They also don't really need to vote either way, these weren't close votes at all. So if they have something else to do, they don't really lose anything by skipping these votes.