Why would any employer agree to that?
You're absolutely right that companies would be better off in attracting talent by revising portions of their contract. You're absolutely wrong to suppose that they're necessarily going to do that.
You just need to mention it when they give you the agreement to sign. By the time there's a piece of paper printed out with your name on it, they're committed. And on the other side of the table from that piece of paper is the guy who hired you. A reasonable person that can recognize a silly clause when you point it out.
Just cross out the paragraph, initial next to it, then sign and hand it back. If they want to make a big deal about it, it's them making the big deal, not you.
Libertarian arguments like this which stress contracts often seem to fall flat by the following reasoning:
1 We seem to agree on what the desired outcome is.
2 If contract theory fails here, the outcome is undesirable.
3 If contract theory succeeds, the outcome is identical to what we would have achieved with legislation.
4 A legislative solution doesn't carry with it the uncertainly that a contractual solution would.
∴ Legislation seems preferable.
You have to know what outcome you want to effect of course, and be reasonably sure your legislation will achieve that. In this case we see (or at least strongly suspect) that when companies are not allowed to own IP produced by their employees independent of company resources and unrelated to their business, the financial benefit to the employee and the benefit to society in economic growth far exceed the benefit to the company (in most cases this benefit is zero). If there were a wider range of solutions I would not support legislation (or possibly I would support narrower legislation), but that doesn't seem to be the case. So I support legislation.