How do you feel about people who don’t tip waiter or waitresses

About once a week me and my husband go out to dinner with a couple of friends who are also married. We always go to a different dine in restaurant sometimes a fancy expensive one sometimes somewhere cheaper like Chili’s. We always ask for separate checks so we pay for our food they pay for their food. Well every single time they don’t leave a tip and it’s makes us feel like we should leave a larger tip instead of a tip based on just the percentage of our meals. The first time we went out we immediately noticed and said something on our way home (just me and my husband) “did you notice they didn’t leave a tip”.

Then we soon realized it just wasn’t that one time it’s every time they don’t tip the waiter. One time my husband said something to him since we all order the same thing and our totals were the same. My husband said “how much should we tip the totals $22”, the friend responded “the service wasn’t that good so I’m not tipping”.

(We were thinking you never tip and there was nothing wrong with the service food came quickly was good and waiter did a good job checking on us it was actually great service)

So what do you think about people who never tip the waiters at restaurants?

Edit- also they make more money then us and are always bragging about all the expensive vacations and clothes, electronics etc, that they buy. So it’s definitely not a money issue. Also most of the time their bill is double or more then ours because they always order a few cocktails each and we just order soda or water since my husband doesn’t drink and I prefer to not drink at restaurants while I’m eating.