It's just a cultural thing. Like certain group(s) asking for a free glass of water*, or constantly going to the toilet to was hands*. Certain groups have their own little ways onboard an aircraft - and clapping after landing is one of them.
Only time I've had it on a flight to the UK, with about 95% British people onboard was a landing during very high winds. Could hear a pin drop during the approach but just after touchdown the whole cabin clapped.
I've noticed a few times people are very unhappy about people clapping after landing - but it's just one of those things. It happens. I don't really have an opinion on it.
;I know they're not clapping at me as a flight attendant but it makes me smile maybe it's a way of thanking the flight crew who (90% of the time they never see) as us mere flight attendants normally get a "thanks" when getting off the plane - unless an ignorent pax can't even show us the common courtesy of saying "thanks", "bye" or something along those lines.
*Before the cries of "racist" come - I am in no way, shape or form implying that these traits are unwelcome or inappropriate and are simply an observation.