I believe the reason is you can’t really dissipate more than about 8W in a standard bulb form factor, while keeping the electronics not too toasty, in a way that is safe and cost-effective.
The corn cob form factor could allow more watts if the LED strips were directly exposed to air instead of being inside a polastic diffuser, but for that to happen you need to be really confident the power supply is well isolated.
There are 15W bulbs but they are larger.