The sentence: "Giraffes are so tall they can grab leaves from branches other animals can’t reach." is almost perfect! It is clear and grammatically correct.
To make it even better, you could add a comma after "tall" to separate the clauses, like this:
"Giraffes are so tall, they can grab leaves from branches other animals can’t reach."
This comma helps the sentence read more smoothly. But overall, your sentence is great!