I think because most of these sites are dominated by user uploads, it doesn't really help to have a great tagging system, because inevitably you end up with a huge library of videos with insufficient/wrong tags. Arguably the majority will have shitty tags, lots of misspellings, etc. The consequence is if you did an AND search with 6 tags say, you'd end up exclude most if not all the results that would match your query if properly tagged.

Weeaboo sites seem to be an exception for some reason, probably because the average community member is obsessive/hoardy enough to give a shit about tagging at all.