My credentials (since this entire thread seems to be a credibility contest):

Former regional FO
Current major airline new hire
Millennial

There's no valid excuse for not tipping. "I don't carry cash" is just a dumb argument. Start carrying cash. Problem solved. I get a pack of fifty 1s from my local bank and store the extras in my lunch bag. It's easy.

"They're just doing their job" is a dumb argument. You're just doing your job also, but do you enjoy it when people compliment your smooth landing? They wouldn't have to do that. You wouldn't *have* to tip the van driver. But it's a gesture he enjoys. Spread some enjoyment. You don't like it when people make snide remarks about your crappy landing. I bet the van driver doesn't like it when you sneak off with your bags and no tip.

"They make plenty of money already" is a dumb argument. Even as an FO, stop kidding yourself. In this day of upward salary pressure, the guy who just hefted your bags makes at most 1/2 of what you make. And plenty of people before you tipped when the wage differential was much smaller.

"I don't make enough money to tip drivers" is a dumb argument. If you eat anywhere other than your kitchen or out of your lunch bag once a month you're lying to yourself.

"No one else tips" reflects more on your employee group than on the societal norm. I've never seen a major airline crew not tip. I very very rarely saw anyone from my regional airline not tip. If your group is flaky on tipping I'm sure the van drivers have noticed it more than I have.