It always intrigues me how most instrumentalists think so little of singing. That isn't to say they don't enjoy the sound of singing. It's to say that they think having facility on their chosen instruments somehow qualifies them to also showcase their amateur voices on gigs. To them, singing is an afterthought; a token activity to pile on top of their "real" work.
As someone who has spent a *lot* of time performing as both a bassist and a vocalist, I can confirm that both pursuits require practice, focus, attention, and passion. As someone who teaches instrumentalists, singers, and people who attempt to do both, I can confirm that a great many musicians try hard to ignore this fact.
Many express the idea that studying singing isn't as serious/noble/intense/difficult as playing an instrument because "everyone already has a voice". While it's true that people tend to have years of experience with their voices by the time they try to sing, it's like the idea that the presence of a piano in your childhood house will somehow make you a gifted pianist the day you finally decide to sit down and press a few keys. Some seem to believe that singing is something that "just happens", and are baffled when they can't duplicate a note by simply opening up and saying "ah".
For any point one could make about how singing is "easier" than playing an instrument because there are no external strings, keys, sticks, or valves to master, I could play for you a passage on the bass that I've reduced to mindless muscle memory. Vocalizing this same passage, however, would require cognitive thinking and intention. A common phenomenon is the player who thinks, "If I can play it, I can sing it". The opposite is true, and observing a student who refuses to accept this is… comedy.
The truth is that singing IS easy... for people who are already good listeners. A good listener can be told, "Listen to the sound of your own voice"... and will just do it. But many people get quite far in life before they ever need to confront what listening to something truly involves, and the idea of having to listen to a sound that is emanating from their own throats is a grand mystery; a paradox; a trick question; confusing; terrifying.
Once a person acknowledges the sounds they can create, it's time they take responsibility for those sounds. This moves us into 'Terror, Pt. 2', also known as, "I AM SO UNCOMFORTABLE WITH THE SOUND OF MY OWN VOICE". It's tough to devote attention to learning an instrument when you feel humiliation every time you make a sound, and this is a real problem for singers. If you're learning to play bass, and your tone sucks, you can place the blame on your inexperienced hands and make the excuse that you're new and need time to practice. Failing that, you can go out and buy a different bass. Too many people think they are 'stuck' with the voice that comes out of their faces the first time they try to sing a note. They can't go buy a new voice, and they feel so self-conscious about the one they've got that they start developing neuroses. This is usually the point some hapless young dude has gotten himself to by the time he asks for my help.
If you want to sing, you have to acknowledge that your voice is the sound of an instrument, and that instrument is your body. Most of the moving parts are inside, so you have to learn to play an instrument that you can't see. It's going to make you a better musician all around, but it won't be a quick process if your attitude is that you'll "just start singing".
A better beginning is to just start LISTENING. If you do, the rest is much easier.