There are two reasons:
1. Manufacturers should consider the battery usage of their products. 4G is temporarily less power-saving than 5G. This is a thing that the manufacturer takes into account the customer's experience. The same manufacturer does not only do this, but also two other menus.
As for tariffs, because most 5G networks currently provide different network speeds according to the level of the package, I don't think there is such a thing for tariffs, and manufacturers generally do not consider this matter.
2, 2G, 3G, 4G have been used stably by default, so no switch is provided. As for 5G, more developed first-tier urban areas have been covered, but most ordinary areas have not yet been fully covered. When the 5G signal is relatively weak, turning on the 5G switch to use the network speed is slower than turning off the 5G switch and using 4G.
I tested a signal. The difference between 5G and 4G in general places is actually not much different in normal browsing. If the 5G signal is poor, 5G will be slower than 4G. You can see the picture:
Therefore, the manufacturer chooses the switch, so that we can use it more comfortably and choose the most suitable way.