People really underestimate the difficulty of playing a violin well.
The instrument is held in a quite unnatural position. Beginners must learn to become comfortable with the contortions necessary. You hold the violin up by squeezing it between your collarbones and your chin. Your hands/arms are to NOT provide any support.
You have no frets on the fingerboard with mere centimeters or millimeters of gap between notes. You must be absolutely precise in your placement at all times at all tempos to make sure you are in tune. Due to the nature of most classical music, you are constantly shifting your hands up and down (forward and backward) on the fingerboard, thus eliminating the ability to "anchor" in one place. Only hours of dedicated practice can develop the muscle memory for such precise placement.
These placements must be done very quickly. A full 4/4 measure of sixteenth notes at 120 bpm means you are placing a finger down approximately once every 0.125 seconds.
Now, while all of this is going on with your left hand, your right hand is manipulating the bow to draw the sound out of the violin.
You must, at all times, draw the bow across the string with the exact amount of pressure, speed, and angle to produce a clear sound. If you are playing a full 4/4 measure of sixteenth notes at 120 bpm all separated (so a different stroke for each note), you are also moving your bow every 0.125 seconds. At the exact pressure, speed, angle and position on the violin. Be wrong in any of these and you will not produce the desired sound.
Often times, due to the rhythm of the piece, you will be moving your bow hand in a pattern very different from the left hand. So you are effectively doing two very different things at the same time. For example, if in that 4/4 measure described above, every group of 3 notes were to be played by a single stroke before changing directions, you are now taking an action every 0.125 seconds on the left hand and an action every 0.375 seconds + one extra 0.125 second stroke at the end.
Now do this while reading sheet music, listening to your fellow ensemble mates to balance sound, watching a conductor to match tempo, express emotionally the intent of the piece.
So then, one must commit enough practice to make all the necessary motions to produce good sound an act of muscle memory. So that there is no conscious thought put into expressing anything the music needs. Thousands upon thousands of hours of drills so the cognitive focus can be on musical expression. To make it as effortless as speaking.
Only then can one sing with the violin. Child prodigies aside, it seems like it takes approximately 3 - 5 years of consistent training before one plays at a level considered "adequate". Elite orchestral players require nearly over a decade of training just to be basic orchestra members. Virtuosos and stars report practicing every day for 4+ hours and will note that any extended break will result in rapid degradation of sound quality and motor skills.
It is a very difficult instrument with an incredibly high skill ceiling and a very long tradition of virtuosity.