For the Japanese, the needs include maintaining a credible deterrence against Russia, China and North Korea. China being the biggest military threat.
OK, so you can choose between, say, 100 F-22s or 300 of some off-brand aircraft. When figuring the total cost of ownership you have to factor in the need to recruit, train and deploy 3x more pilots. Maintain and provide spares for 3x more aircraft, as well as recruit train and support a larger number of maintenance and logistics personnel. You also have to buy, store and maintain as many as 3x more weapons and stores, including training rounds, etc. You have to purchase and store up to 3x more fuel. You likely need to buy more tankers to support the operations of a larger fleet. You need up to 3x more basing area.
So is the value proposition of buying more cheaper aircraft as good as you think it is?
So, if you put a squadron of F-22s in combat, fully integrated with their AWACS to leverage the Raptor's unprecedented electronic integration, exactly what do you think has a chance of touching it? Of course no plane is untouchable, but...wait a sec...no F-15s ever lost to enemy aircraft...on second thought maybe untouchability is achievable.