Because Advice-Only financial advisors only sell their advice, they typically don’t require any minimum assets or minimum income.
Because they don’t charge a percentage of your assets, how much assets you have is irrelevant. Financial planning for someone with $5 million in assets and multiple types of accounts may cost more than planning for someone starting out, only due to the difference in complexity.
Some advisors are selective because they like to work with certain type of clients, similar to how clients also choose advisors they like. It has little to do with how much assets or income you have.