Yes, the issue is that build scripts are typically written and maintained by humans (as opposed to programs). A more pleasant syntax is one that would be easier to read and maintain.

I think if you want a simple serialization format to replace Make's syntax, s-expressions would be a better fit.

You can use mk, the successor to make, which does not have the TAB requirement (you can use TAB or spaces interchangeably).

Regarding JSON, it is more verbose than Make. You need to use quote around strings, commands, everything. Compare:

foo.o: foo.c gcc -c foo.c

with

{ 'foo.o' => ['foo.c'; 'gcc -c foo.c'] }