If it were that simple, they would’ve done it already.It make sense for MacBook Air because there were no other Computer ( not just the Mac ) that was this thin. The actual technology to make a Laptop that thin was expensive. Not only was MacBook Air the first MacBook to have SSD, which was expensive at the time, it was also one with Ultra Low Voltage CPU specifically designed by Intel. Along with all other innovation required to make it thin. It was expensive because it is a technical marvel.
There is nothing technical about making an iPhone thinner in 2025 other than making different sets of trade offs.
As far as I know, there really isn’t any mainstream smart phone that is M4 iPad Pro or even the last iPod touch levels of thin.
Even the thinnest iPhone is still almost a MM thicker than the latest iPod touch, and 2 MM thicker than the current iPad Pro.