What they actually mean is rather "these two things are very dissimilar", or "these two things are unequal".

I guess in most situations "cannot be compared" could be replaced by "cannot be equated", with less lingual inaccuracy and still the same message conveyed.

To come to the conclusion that two things are very dissimilar, very unequal, one necessarily has to compare them. So it's rather odd to come up with "cannot be compared" after just literally comparing them.

For example, bikes and cars. We compare them by looking at each's details, and finding any dissimilarities. They have a different amount of wheels. Different propulsion methods. Different price, and so on.

When this list becomes very long, or some details have a major meaning which should not be equated, people say they cannot be compared.

An example with a major meaning difference: Some people say factory farming of animals and the Holocaust are very similar, or something alike. Others disagree, presumably because they feel wether it's humans or animals being treated, the motives or whatnot make a difference big enough that the two should not be ~~compared~~ equated.

Can you follow my thoughts? Are 'dissimilar' or 'unequal' better terms? I'd be especially interested in arguments in favor of 'compared'.