Why do HEPA air purifiers need replacement filters?

I could air purifiers do not require replacement filters to find, but I could not find a HEPA air cleaner that does not need a filter option. Not only do they require the replacement of the filter, but filters are very expensive.

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personal air purifier (5),HEPA Air Filters (2),microlux air purifier (2),aIR fILTER Purifier Filters (1),idylis filter replacement (1),replacement hepa filters (1)