They come up with ideas, and when special interests and other actors end up completely breaking the intent by changing certain pieces, they still proceed with implementing the broken legislation for public perception reasons of "doing something".
See the Affordable Care Act for example. Obama wanted a public option to keep insurance companies in check and help drive down the price-gouging from the providers. Joe Lieberman, as the 60th Democrat in the Senate, was able to block that by pulling his vote, allowing for a fillibuster. He wanted the public option removed, and he got his wish.
Obama pushed the law through anyway, despite the fact that this change completely broke the design of the law. Insurance companies now saw it as a subsidy for new customers, which is what it was. The broken design failed to reign in costs, which made the subsidy provided by the government inadequate to make it affordable to the working-class, resulting in the law effectively being a Medicaid expansion and nothing more. The prices charged by insurance companies through the exchange were too high to ease the pain of people too rich for Medicaid but too
Obama originally treated it as a huge victory. It wasn't. It was broken on delivery. But hey, they "pushed something through" and "won."
Now we are getting honest assessment of it as it implodes, and the Democrats are talking about single-payer again.