Why do UI/UX designers not help without charging?

It's also a good example of why quick design help is hard, because there is no way for anyone but you to know what would be incrementally better or not for your users. All an outside designer can do is suggest things that might be interesting to test. You still have to try them and measure the change over time. A given designer has no specific knowledge about your market, and something can be well-designed in general but still have a lot of potential for market optimization.

( In fact, a colleague runs a service that is just that: he'll try something new for you, every month. https://draft.nu/revise/ )

Since only you have the data on your MVP, start there. Put together your customer lists. Make sure you understand their demographics and their usage patterns and why they purchased and what they were considering instead. Make sure you have ways to contact them for research interviews and if it's possible to go on-site to see them use your product live. Document how your competitors present and discuss their products.

Then, pick something. One small thing. A landing page. A particular form. A headline. Copy for a Google Adwords ad. Something you can measure your current performance on, and measure a change to afterwards. If you're not high-volume, this might be harder to find than you think, but go through the exercise of figuring out what you can collect enough data on now, and what can wait to change until you can measure it later (or what you don't care about measuring).

Then, you can take that one thing to a designer and say, "I'm looking to experiment with different changes to . What are some things you think might be interesting to try? Anything goes."

You're not asking them to do any work. You're not asking them to stake their reputation on the line. Your language ("experiment," "different changes (plural)," "interesting," "try," "anything goes") all says you're willing to respect their time and level of engagement and also that you're prepared to handle their advice properly, that you're not looking for a silver bullet, you're looking for all of the lead bullets.

Say you ask for help with a landing page. A designer might suggest some copy or visual design tweaks for clarity (specific changes), but also suggest you test different headlines (without providing any), and also changing your format from short form to long form or vice versa (definitely without providing any content to you, but maybe they would be interested enough to sketch out some varied layouts). These are all great ideas. You should read up on A/B testing and test all of them in isolation, and also all together. You should test big dramatic changes even more frequently than you test small copy changes, because you don't want to be stuck looking for a local maximum. A UX person might also suggest ways to find out other things to test, research questions to ask, and suggest books or other resources so you can collect that data well.

A startup that came to office hours that well-prepared would basically almost not need design help at all.

If you don't have the money to afford quality help, and instead you have to beg, then you're not where you think you are yet.