Why is proving "parties' views are simulatable" enough in semi-honest model?
I am having a hard time understanding your argument why you think this could be simulated. However, here is an explanation of why it could not. 

Assume Bob is honest and has input $y$, now we have to simulate the view of Alice. As the simulator, as you state, we only get the input and output of Alice. I.e, we only know $x$ and $f(x,y)$. We only need to simulate the one message in Alices view, i.e., the first message where Bob sends $y$ to Alice. Since we do not know $y$ the best we can do is to randomly guess at some $y'$. Now the probability that $y' \neq y$ is going to be high, so the simulation is going to be clearly distinguishable from the view of Alice.

What might be confusing you is that, when we do the simulation we do not know the inputs of the honest parties. But when we try to distinguish the real view from the simulation we know all the inputs.