The controller adds cost to the device, and it is easier to do charge control with other methods like voltage level monitoring. Charging a battery inside the device allows as well the convenience of being able to charge during the day when you want to use the phone or whatever, while doing the actual recharge later when it is convenient for you to do so. MPPT adds a complex inverter, doable at low power, but it will probably double the cost in a small device, where you already have high cost items like a solar panel and a battery using a majority of the production cost.
