Why shell commands in makefile usually start with at symbol?

Answer:

Normally make prints each command line before it is executed. We call this echoing because it gives the appearance that you are typing the commands yourself.

When a line starts with @, the echoing of that line is suppressed. The @ is discarded before the command is passed to the shell. Typically you would use this for a command whose only effect is to print something, such as an echo command to indicate progress through the makefile:

Source: https://web.mit.edu/gnu/doc/html/make_5.html.