> Let's say, a restaurant has 20 tables and initially has 5 servers.

This is already overstaffed and you are doing the math backwards. The place with 5 servers in the USA almost always has runners as well making total floor staff 8-10.

The comparison is not a mythical restaurant with 20 servers for 20 tables. In Europe a typical place with 20 tables will have 2-3 servers and no runners. We go from 8-10 floor staff down to 2-3.

Hopefully it is clear how 5 servers dedicated to 4 tables each and backed up by runners can do a lot more upselling than 2-3 staff doing team service. And they don't need to because their salary isn't dependent on it.

Locking down 1 server to a strict set of 4 tables is very inefficient as well especially when one section is slammed and another is half empty.

You will find plenty of study of tipped wages and restaurant economics but they generally don't use words like "overstaffed" instead they say things like analysis found that owners systematically offload slow day risk to tipped employees.