Text does have the advantage of incumbency. When programming languages were first developed, we had teletype terminals which only accepted a limited character set and no graphical input devices, and CPUs which could only execute a single instruction at a time. That constrained the languages people could develop. After adding decent graphics terminals and multi-core CPUs, and noting that most algorithms have instructions which can be executed in parallel, implementing programs as directed graphs makes a lot more sense.

Text-based languages can still be very different from one another. There's only the appearance of similarity now because most programming languages, and all mainstream ones, are descendants of ALGOL60, influenced to a greater or lesser extent by Smalltalk and Lisp. There's almost nothing in common between Prolog and C, for example.

How can you read any program without the (source) code on hand? The same problem occurs, for example, with Java byte code.