Seems like there is a lot of manual adjustments to be made when beaming someone up (maybe fewer when beaming someone down) -- the transporter chief seems to eyeball the subject and move controls accordingly. This looks like an extremely iffy process and if those adjustments are made too fast or too slow, what happens? Why is the entire process not completely automated?
Frankly, transporter technology is one of the leading causes of problems in space exploration. Poorly-executed transports can be gruesome affairs. Interference from the environment can lead to any number of medical or existential issues, such as fusing people into new life forms, duplicating officers, creating phased-out ghost officers, accidentally turning officers into children, accidental time travel, or stranding officers in a parallel universe.
These problems have existed from the very beginning.
Despite all this, Starfleet officers routinely act as if there is no problem. Even officers who should know better. Geordi LaForge once told one of his officers, "Reg, how many transporter accidents have there been in the last ten years? Two? Three? There are millions of people who transport safely every day without a problem." This is particularly egregious, considering LaForge had already encountered several transporter accidents during his time in that post.
As for the question of manual controls, this seems to be part of a larger tendency within Starfleet to resist automation entirely. There are a few possible explanations, but there's not a definitive answer. My personal favorite is that Starfleet massively pulled back from automation after the M-5 incident resulted in hundreds of deaths. Another is that just a few years earlier, a rogue AI coopted a fleet of starships and caused a rather large conflict and a time travel event. We have also learned in recent years that there is a cult of Romulans who have been deliberately sabotaging AI efforts for generations out of a fear that AI will inevitably lead to extra-universal AI monstrosities annihilating all organic life.
It's probably a combination of these factors.