It's hidden because it's an advanced feature and bad coin selection can actually hurt you in the long run on fees and with some other traps (e.g. making dusty outputs, etc.)
I completely agree with this. Wasabi has three different coin selection modes but they are hidden by default because the automatic coin selector does a much better selection than most users do. That's possible because the wallet knows who knows about each of your UTXOs and then it can select automatically UTXOs that are already known by the payee or, if that's not possible, it allows you to decide who can know about the payment transaction. Wasabi also tries to avoid creating change as Bitcoin Core does and suggests different amounts close enough to the attempted payment to avoid it.
However, there are cases where you know exactly what you want, like spending one very specific UTXO as a whole (1-in, 1-out) or a set of specified coins as a whole. In those cases, the user should be able to select the coins themselves. Wasabi allows you to do that too but it displays who is going to be able to track that transaction and alerts you in case you are mixing private coins together with non-private coins.
Finally, there are other cases where the user thinks they can do better than the software and wants to see for themselves the set of coins it will use and change it. This scenario is not absurd; in fact, users feel more internal peace/safety if they select the coins. Wasabi, for example, allows users to select the coins but the software will inform you if your selection can be improved, if it ruins your wallet's privacy, who will be able to track you, and whether change can be avoided or not.
I was against manual coin selection for a long, long time until I realized that having no control over the coin selection, even if the wallet could do it a million times better than you, creates a disgusting feeling of uncertainty and fear.
Summarizing: Automatic coin selection should be by default. Assisted manual coin selection should be there but hidden. Unassisted manual coin selection should be available too but really hidden.