I keep track of when I have an empty seat next to me. It happens maybe 1 or 2 times per year out of 60 - 70 flights per year. 2020 was the exception both in terms of number of flights and rate of empty seats.

I will say that I almost always pick an exit row seat when I'm in economy because I like the extra legroom. The middle seat might look to be empty until a basic economy or standby passenger drops in at the last minute.

Back in the "old days", the gate agents might try to leave seats empty next to GS and 1K passengers, but I'm pretty sure that doesn't happen anymore.