From my experience in the industry and observations as a patron, I think part of this is server politics as well. Since a solo diner will usually run up a lower tab than a party of two, it will usually result in a lower tip from that table. Many restaurants also incentivize their servers based on how much they sell each shift (I.e., whoever sells the most on Saturday night gets to look after the VIP table the following weekend)... so if a solo diner will spend less, it hurts that server’s chances. Of course, absolutely none of this is the customer’s problem. But it’s a pretty consistent theme that I’ve seen both as a solo diner and as a restaurant worker, unfortunately.