Fuel isn't going to move if you don't have a pressure differential.

Exhaust will go back in to the gas tank without a valve (injector).

Need to control the amount of fuel on each combustion cycle.

That's why.

Tank is in the back. The injector only works properly at elevated fuel pressure otherwise it would just drip out. Like a shower head with no pressure, it would just dribble out.

Valve lets in a mixture of air and fuel, not just fuel, and allows compression to happen. If that wasn’t the case you’d have to have a mixture injector that closes during compression and explosion. Without it, the explosion would expand in the intake runner instead of pushing down the piston.

Thanks. So does the injector get fuel on the back of the valve when it sprays?

Unless it’s direct injection, yes. It sits there for a little bit mixing up with the air. Also the hot valve vaporizes the fuel a bit more.

In direct injected engines, the injector shoots on the piston.