if they think they have no case or are crazy?

As a starting point, it would be unethical and unlawful to prosecute a case that a lawyer knows has absolutely no merit. In federal court, that standard would be set forth in Rule 11

(b) Representations to the Court. By presenting to the court a pleading, written motion, or other paper—whether by signing, filing, submitting, or later advocating it—an attorney ... certifies that to the best of the person's knowledge, information, and belief, formed after an inquiry reasonable under the circumstances:

(1) it is not being presented for any improper purpose, such as to harass, cause unnecessary delay, or needlessly increase the cost of litigation;

(2) the claims, defenses, and other legal contentions are warranted by existing law or by a nonfrivolous argument for extending, modifying, or reversing existing law or for establishing new law;

(3) the factual contentions have evidentiary support or, if specifically so identified, will likely have evidentiary support after a reasonable opportunity for further investigation or discovery; and

(4) the denials of factual contentions are warranted on the evidence or, if specifically so identified, are reasonably based on belief or a lack of information.

With that said, in my experience, yes, some lawyers won't take on a client who has plenty of money to pay because the case is a dead loser, but can still be brought without violating Rule 11. No matter how many times a lawyer tells the client she's going to lose, there's a good shot she gets mad at the lawyer--and then doesn't pay the bill--because she thought she had a slam dunk, or she only lost because the lawyer didn't do his job properly.

And along those same lines, some clients and/or cases are headaches that aren't worth the money.

And some clients and/or cases just violate your personal ethics and/or morals (yes, lawyers have a soul too sometimes). For example, would you take the money of a serial child abuser to defend him in a civil case?

Of course, all of this varies from case to case, and lawyer to lawyer.