One reason why people might not fit towing mirrors is the difficulty, or impossiblity, of finding ones that will fit. I gave up, and ended up making an aluminium adaptor for a pair that did not. I suppose i will now get arrested for "making an unauthorised modification to safety equipment" or some such 'elf and safety BS.
Fortunately I am quite handy at this sort of thing, but I would think it would be beyond many others who are not equipped for metalwork with a shed, bench and tools.