Probably due to the seating areas are still shut down and people sit in their cars and use it ? Family Mart is pretty good. Once you sign into it, it remembers everything and when you go back into the next time all you do is turn on your phone and it is automatically turned on.
Some low volume McDonald's experimented with free wifi to get customers to come in, but with more people finding it cheaper these days to buy a bento box for under 500 yen, customer count is down so the 2 out of 3 McD's in town here no longer offer free wifi.
StarBucks in town has free wifi, but at 400 a cup of mud, forget it. I go to McD's that still has the free wifi and good ( I think, Vietnam export coffee ) for 100 yen , tax included with background music.