Well for one, hieroglyphics isn't a language. The reason that English speakers have trouble understanding old English is because languages constantly change through time. When millions of people speak a language, the language will always change to accommodate the needs to the people using them. Hieroglyphs is a way of representing a language and not a language itself.

Hieroglyphs also went through some pretty important changes during their almost 2000 year reign. The introduction of hieroglyphs appeared in the early dynastic period where lower and upper Egypt was united under one kingdom. The problem with hieroglyphs is they are extremely time-consuming so heiratic script began being used that simplified hieroglyphics and slightly resembled the original look. From the 7th to the 5th century BC, demotic scripts began to be used by the Egyptians that was based on the heiratic script (which was again based on the hieroglyphic script). In, 30BC the [Romans took control of the Egyptian territory](https://en.wikipedia.org/wiki/Egypt_(Roman_province) and made Latin the language of elites, decreasing the role of Egyptian hieroglyphs and it's related written text. the biggest change in Egyptian hieroglyphs occurred after the capture of Egypt by Alexander the Great in 331. Once the Greeks maintained control over Egypt, most of the elites again moved their once nationally-exclusive language in both written and spoken form to that of Greek. It was during this time in which Coptic became the leading form of written text in Egypt that was derived from Greek text but also had taken some influence from Heiratic (and thus other ancient scripts).

To make a long text shorter: Hieroglyphs did change as much, if not more, than language. The problem is, those changes are not well-represented in popular culture and are so different that one may not even know that it was a derivative of an ancient text.