Because you're wasting your $500? The lawyer will want to make edits to protect you or they will tell you not to sign. The builder will say sign it as originally written or get out of the way and the next person in line will sign it.Builders purchase contracts are completely, 100% one sided. If buyers on mass refused to sign then those contracts wouldn't exist.
I also wonder how many potential buyers actually have their lawyer look at those purchase agreements and fully explain them.
My guess, most don't.
Hey why spend $400 -$500 to understand and protect yourself from the biggest purchase of your life.
If you want to try to make money on pre-construction housing, you accept the crap contract and hope for a gold-plated return. Alternatively you buy existing housing stock and have a more meaningful negotiation but more often than not achieve a lower rate of return over a short period.