You do not necessarily need to be backed by sponsorship to grow your channel. Some people use the community or fans to help them grow. For example - It's obvious you won't have resources to get every single motorcycle to try, and maybe the shops/manufacturer is not willing to let you try them, you can turn to your community, or fans, some would be willing to lend you their motorcycle to try and stuff. Yes, it is risky because that is someone's property that you will be responsible for. Just pointing out that sponsorship is not mandatory to grow or be big.
On the ROI aspect, that is absolutely true. No company would drop money/product on you without knowing what they gain from it. It's a business to them, not charity. Sponsorship comes with some sort of T&C which you are now obliged to honor, so even getting sponsorship is tricky because you need to commit, there is no free meal.
That being said it's not all doom and gloom, if you find the right partner that can work well with you, the sponsorship might be just perfect for both sides.
Whatever it is, do your checks before signing up for anything, we always like to think of the fun part - free gear, free bikes, free, free, free; but forget to also note the responsibility and commitment you have to uphold.