If you lock in a fixed rate and the rates drop, you can always refinance and lock in a lower rate.
If you stretch your budget to afford a house at a low variable rate and rates climb, you're in trouble.
I agree, the budget has to be able to absorb increases but the fatc is the market that sets rates wants a premium for fixing the rate long term. The investors are not stupid, they project what the future will bear for rates and will demand premiums for the risk of lending for longer term at fixed amounts.
In other words the fixed rates provide a guarantee, and that guarantee cost money.