Because the verb stem has an alpha at the end.
You can also say βοηθάει if you want.
These alpha verbs (βοηθάω, αγαπάω, μιλάω, ...) have double forms for most forms:
- εγώ βοηθάω, βοηθώ
- εσύ βοηθάς
- αυτός/αυτή/αυτό βοηθάει, βοηθά
- εμείς βοηθάμε, βοηθούμε
- εσείς βοηθάτε
- αυτοί/αυτές/αυτά βοηθάνε/βοηθούν(/βοηθάν/βοηθούνε)
The way I learned it, there are three main groups of verbs: those that have the accent on the stem (probably the vast majority) such as έχω, θέλω, βλέπω, ...; those that have the accent on an ending in -άω -άς ... such as αγαπάω, μιλάω; and those that have the accent on an ending in -ώ -είς ....
It did, in Ancient Greek (where it did not have the meaning science exactly, but "the good knowledge of something"). In Modern Greek, it means "science" only (but note that science contains the meaning of knowledge in it; science is the total knowledge about a particular field of research). In Modern greek, there is το επιστητό, that has roughly the same meaning as the ancient greek επιστήμη. Το επιστητό=1) everything humans know through scientific research 2)special knowedge on a subject.