Why would anyone want to buy one unless it will increase in value or has real world value attached to it (for example, if owning one means you can meet and greet that player or the team during the season)?
If not for the two reasons above, why not just take a screen shot of the image they will sell you as an NFT?
Well, in response to your first point, purchasing an NFT can and often does result in 'membership benefits'. There's no reason why United couldn't do something similar. You could either sell off rare NFTs that had special privileges to the highest bidder, or you could take a more egalitarian approach and assign special benefits to certain NFTs at random, so that it became a 'golden ticket' style lottery.
Secondly, we have already talked about the fact that humans are innately drawn to collect things. Your 'why wouldn't people just take a screenshot' is misguided...did you ever collect Panini stickers when you were a kid? If so, why did you bother buying packets of stickers in order to find the rare stickers you needed to complete your album? Why didn't you just make your own sticker, or take a photo/screenshot of a sticker? Why didn't you just go on eBay and buy all the stickers you needed?
Remember, as I said to another poster earlier, collecting is meant to be fun, and we don't have to take the extreme examples of NFTs going for hundreds of thousands of pounds and use those to rubbish the idea. I can definitely see a marketplace for fans to pay £5 for a 'packet' of NFTs, which can be stored in an digital album, with the objection being to collect them all.
I feel like some people are failing to understand perspective of others. Just because you might not be into collecting, doesn't mean others wouldn't be.