It’s not necessary — Fortran didn’t reserve any words, so things like:
if if .eq. then then if = else else then = if endif
are complete legal. This not only makes the language hard for the compiler to parse, but often almost impossible for a person to read or spot errors. for example, consider classic Fortran (say, up through Fortran 77 — I haven’t used it recently, but at least hope they’ve fixed a few things like this in more recent standards). A Fortran DO loop looks like this:
DO 10 I = 1,10
Without them being side-by-side, you can probably see how you’d miss how this was different:
DO 10 I = 1.10
Unfortunately, the latter isn’t a DO loop at all — it’s a simple assignment of the value 1.10 to a variable named DO 10 I (yes, it also allows spaces in a name). Since Fortran also supports implicit (undeclared) variables, this is (or was) all perfectly legal, and some compilers would even accept it without a warning!