Because the PC is offering every spell he knows. The NPC would have to be crazy not to take that deal.Because why would the NPC say "yes" when they could say "yes and give me 300 gp per spell level for the privilege"?
Imagine if the situation were reversed. You're playing a level 3 wizard who knows every spell except magic missile. A level 1 wizard NPC comes up to you and offers to give you magic missile for free if you let him take a look at your spellbook. Why would you say no?Try replacing the word "spellbook" for "wallet", and you'll see why. Knowledge is power, and has value. I won't just hand you power and value unless you give me something of equal value in return.
So, if both feel they're getting enough out of the deal, sure, they can do that. But, if I have four spells, and you have seventeen, you aren't winning on this deal, are you?
What I'm really asking is, if a PC wizard asks an NPC wizard to share spells, why would the NPC ever say no? And, if the NPC would always say yes, then how come every wizard in the world doesn't know every spell in the world?
Sure, but what about low levels?Two wizards get together. They each hand the other their spellbook. Wizard A opens the spellbook and reads "I prepared explosive ruins today." Wizard B casts teleport and hightails it into a magnificent mansion.
Or, to put it more succinctly: it's often too risky.
Sure, but what about low levels?