They look cool but is crystal strong?

Depends on your definition of strength.  They are *magically* strong.

http://darksouls.wikia.com/wiki/Crystal_Magic_Weapon

And then there are glass weapons, which are strong enough to slay White Walkers!

http://gameofthrones.wikia.com/wiki/Dragonglass

But to answer your question, a lot of crystal points were made and crafting them required advanced chipping skills.  I doubt flint would be abandoned for crystal unless crystal had utility.

Perhaps they liked how crystal looks?

Why not both?

Yes, likely both.

http://www.smithsonianmag.com/science-nature/early-bow-and-arrows-offer-insight-into-origins-of-human-intellect-112922281/

Similar to obsidian knives which can be sharper than modern surgical tools due to the structure of obsidian.