a program like any other that just needs to be able to access each websites’ server and display its files right?

In software engineering “just” is often considered a dirty word.

Rendering HTML and CSS correctly is not trivial.

Doing JavaScript to spec also is not trivial.

Doing all your http verb network request stuff is also not trivial.

Plus the interface (probably graphical) is a lot of work.

There’s also probably a thousand other things that would eat up time. Displaying all the different image formats, for example.