You can think of one of the simplest reasons: if you can achieve 40W charging with a 40W charger, why should manufacturers spend a lot of money to research fast charging technology?
Now digital electronic products are very smart, mobile phones, data cables, charging head, the three must agree to charge. For example, your phone only supports 10W, the data line supports 40W, and the charging head supports 40W. Then it will automatically charge at 10W.