Because it is another word and hence groot is not treated as an adjective. Since you are also learning French, in French it is:
- La grande mère - The big mother, but:
- La grand-mère - The grandmother
There also the grand in grand-mère does not follow the gender rules for adjectives because it is a new word.
You could argue that not following the adjective gender rules makes it clearer to know that it is another word with a different meaning. Otherwise how are you going to tell the difference between.
- De grote moeder, and
- De grotemoeder