I want to ask then why many people try to build the gantries with aluminum that it is lighter than steel and also more expensive.
Because aluminum is much easier to work with for most people.

Why many people try to make their gantries as light as possible without of course having compromises with stiffness.
Because they don't know any better.

Finally why nearly every Z axis is made from aluminum and not from steel.
Again, aluminum is usually much easier to work with. You can cut it with woodworking tools, or even very lightweight cnc routers if you take your time. Steel requires different tools, or you need to pay someone to do the work.

The Z axis is one place where lighter weight is helpful. But I think ease of fabrication is still the main reason aluminum is used.