Each state sets how they do elections. Some states like mine have 100% mail in voting so I can do it at my leisure well ahead of election day. Other states force you to do it in person and play god with polling locations by flling Republican areas with then while having little to none in Democratic areas meaning long waits and disenfranchised voters.

it depends on the state one is in. Mississippi probably has restrictive absentee ballot access. (not that I would know, I'm in Minnesota, and my ballot was mailed to me automatically. here, it's something you have to opt into, though.)

I think mail-in vote eligibility for Mississippi is pretty limited.

Also, I'm not from that state, but I recall a while ago there was some sense that the federal government was going to try and mess with votes by fucking with the post office. Some of what they tried failed in the courts, but I decided I'd vote in person to avoid that particular problem, so there might be other people who decided in-person was most likely to get their vote counted.