Imagine the situation. You are a kid, in a group of kids who are all playing the same instrument, recorder for example.
Given this situation, is it possible to hear when you are playing in tune?
Have a play with the following tool. Can you find a pitch that makes what you are hearing sound good?
Now try to do the same thing again using the following tool. Again, can you make the result sound good?
What you will find is that the second tool it is easy to hear when the note you are controlling is in tune. The first tool by comparison sounds bad regardless of what you do.
Why is it easy to hear correct intonation with the second tool, but not the first?
The second tool plays two notes of a single frequency. We can visualise this as a graph as follows.
It is easy to hear when both are in tune as both the fixed pitch the tool is playing, and the one controlled by the slider are a single frequency, with a nice clean spike like above.
When they are in tune, the two spikes like up flawlessly, producing a nice clean sound.
The first tool by comparison simulates a classroom of children playing the same instrument slightly out of tune from each other.
If you graph those pitches you'd have something like the following. A wide spike with a flat top:
When a single player is varying their own tuning, what they are hearing is the sharp spike of their own instrument, combined with the wide 'hill' all of the other players are creating.
In simple terms, when you have a large number of children playing the same instrument, yet slightly out of tune, it is impossible to hear intonation as there is no centre of pitch!
There are two very easy ways to avoid this issue:
Teaching one at a time allows the children to learn to hear intonation in the easiest possible situation.
An instrument with a different timbre allows intonation to be herd even when there are a lot of detuned notes, as such an instrument has different overtones. People can use these to base intonation, even if the fundamental frequency is unclear.