But more to the point, there are various jobs widely known to be totally underpaid and depend on tips. For instance, you tip bartenders in the US even though all they are doing is pouring you a beer. Is it "right"? Of course not. But it's a widely accepted societal norm in many countries, and is an assumed part of that person's income.
Quality of service questions aside though the bottom line of whether or not you should tip comes down to if the person in question is working for the tipped employee minimum wage or the standard minimum wage.
If they are paid according to the tipped minimum and you don't tip based on your ideological view of the appropriateness of a tip for their job function then you are an ass.
It if didn't arrive in good shape, I would be asking for a refund, or at the very least not ordering form that place anymore.
I don't know, it's weird - I currently live in UK, where tipping waiters is defacto standard(and not because they make below-living wages), but I would never ever think to tip a delivery driver. Like....why would I? This makes absolutely no logical sense, just like I wouldn't tip a plumber or a car mechanic for fixing my stuff on time - they are already taking money for the service.
There's your issue. Tipping culture in the UK is very different to the US. Mainly because, as you say, they do not make below-living wages.
But within you example, I still don't see the distinction. Waiters can treat you badly or well - and you tip them accordingly. Delivery drivers can treat you badly or well - and you tip them accordingly.
In contrast, my experience with the delivery driver is limited to the 10 second it takes for them to hand me my food. Literally every single takeaway takes payment online, so for years now my food was already paid for before it was even delivered. So all the driver has to do is give me my food. He doesn't even have to say anything, there's no conversation to be had, "delivery experience" to experience. So why tip?
If you want to be righteous call the manager at the place you're ordering food from and complain, don't stiff the guy delivering pizza for a living.