Why don't ambulances have stretcher ramps?

There's probably a really good answer to this question but I'd like to know.

At the risk of being accused of trying to reinvent the wheel: why aren't stretcher ramps more widespread? I can see a lift being cost-prohibitive, but even some taxis have ramps. And while pushing a person uphill is no walk in the park, I'm fairly certain it beats lifting them. Seems like a sensible solution.