It’s because you’ve been shampooing daily (or however frequently). Shampoo is designed to remove the oil from your hair/scalp, so then your scalp has to compensate (and also you have to add conditioner to put it back).

The way to switch over to no-poo is to slowly increase the number of days between shampooing until it isn’t an issue anymore. Only shampoo when you need to.

Also, try washing with conditioner instead of shampoo (co-wash). The oils in the conditioner bind with the oils in your hair so you still remove some of them, but it doesn’t strip it all. I (a man with thick about shoulder length hair) only co-wash once every week or so. I wash with water daily. This is what’s worked for me. You will have an adjustment period though because you’ve been putting your scalp through so much, it has to get used to that not happening.