It doesn't do that because most startups fail. Selecting for the ones from which you can extract the best terms just gives you more of the failures. Negotiating strength isn't just a proxy for company strength, it's also an intrinsically important business skill. This is one of the more basic illustrations of "adverse selection" you can find.
I'm sure there are VC firms who do scout out pushover startups and bleed them. But then, most VC firms fail to generate attractive returns.
I challenge you to find statistics showing that any VC favors the tough negotiators over the easy ones, or that there's any correlation (let alone causation) between negotiating strength and future success. Yes, it's an important skill, but negotiating with VCs and negotiating with customers are two different things. That particular subset of negotiating skill pales in comparison to things like having a good vision, hiring good employees, or executing the rest of a business plan well. The world already has too many people whose only skill seems to be schmoozing VCs.
I'm sure there are VC firms who do scout out pushover startups and bleed them. But then, most VC firms fail to generate attractive returns.