Some people feel that in being asked to remove their shoes, they are being treated like children.
They may find it obnoxious, having to follow a house rule. They may think it is fine to ask children to take off their shoes, as who knows what they might step in, but they feel patronised to be asked to take off their own shoes. They believe that adults can be trusted to wipe their feet on the doormat.
What I would say to these people is that there are many good reasons why hosts may ask them to take off their shoes. I suggest they try not to think of it as a house rule, but just as a polite request (like somebody asking you to hold their umbrella). These people probably go shoeless sometimes in their own homes, so it should not be too onerous to do this when at the homes of others.
The fact is that wiping your feet will not remove all the dreadful stuff that your shoes pick up. You may try to be careful what you step in, but a lot of the worst things, like lead or weed killer is unseen.
Other people may feel that removing their shoes makes them feel childish. They do not feel terribly grown up skipping around a house in their bare feet or padding in their socks. What I would say to those people is that they should bear in mind that in many cultures, going shoeless indoors is the norm. In Japan or Sweden, nobody thinks it childish to be in socks or bare feet. There are plenty of situations in the UK where adults will be without shoes; such as on the beach or in a Yoga class. I suggest that when they visit shoeless homes, they ought to bring some slippers to wear.