More so than the games themselves in July, you have to consider all of the training and qualifiers that the athletes must participate in leading up to the games. All of that has been put on hold, making it impossible to properly prepare for the games even if the virus were to somehow be totally eliminated by July.
I work for a major broadcaster, we have a lot of equipment to ship in 2 weeks weeks time, equipment that still hasn't been configured, and indeed that's spread around multiple buildings across the country. We were expecting this, but we also had to be sort-of preparing to ship kit on time
Japan’s preferred date in October would be much more pleasant for athletes, spectators, and tourists, but there was a lot of pressure from US broadcasters to schedule it for July.
July is after the NBA and NHL finals. It is before the NFL preseason. MLB is at the midpoint of their long season when it is late enough that the excitement of the new year has warn off and it is too early for the playoff race to reinvigorate fans. July is generally the slowest sports month in the US.
Meanwhile in October, the NBA and NHL both start their seasons. The NFL and college football are both going strong. MLB is in the middle of their playoffs. It is one of the busier sports months.
This is also just the sports calendar. Other aspects like the school schedule and traditional TV calendars also point to July as an ideal month.
Not only are Olympics no longer the festival of human achievement but lets have commercial interest dictate when they should take place, even if it makes no sense for the host country. Perfect.
(Anyone who visited Japan at that time of year will tell you the weatjer is pretty brutal, with very high temperateure and high 90%+ humidity.)
Why the negativity? Would you prefer to ignore entertainment context and only think of the weather in the host country?
Commercial interest has a direct correlation to the number of people who tune in. Even if money wasn't a factor, the number of people watching should be a core metric.
There was no other possible outcome.