…if they are supposed to be Buddhists?
This isn’t unique to Japan of course – try getting truly vegetarian food in Thailand!
One possibly relevant factor in Japan is in the animist collection of beliefs that is Shinto, plants are also gods and so it is just as bad to eat a banana as it is to eat a cow. For that reason, you traditionally thank/ apologise to all food for giving up its life for your nutrition with an “Itadakimasu” and a praying gesture. Another is that extremism in diet is just as frowned upon as extremism in religion (the reason why 99% of Japanese will respond to “Are you religious?” as if it’s an accusation), so cutting down on meat is sensible and cutting it out is just kind of weird.
The other possible reason is simply that Buddhist orders in Japan have always told their public what they want to be told, so that the priests can fill the coffers which they will pass onto their sons (marriage and children being another thing that isn’t traditionally associated with Buddhism!) Again, Thailand jumps instantly to mind as proof that money grubbing Buddhist priests aren’t soley a Japanese thing, though…