Why shell commands in makefile usually start with at symbol?

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.