Why does the read command in bash work with a herestring but not with piped output?

I'm trying to understand how the bash read command works under the hood. Given that it expects its input to come from standard input, I was surprised to find out that piped input does not work as expected. E.g.

### Pipe scenario

echo "1 2 3" | read -r one two three
echo "one: $one, two: $two, three: $three"
# output:   'one: , two: , three:' 

### Herestring scenario

read -r one two three <<< "1 2 3"
echo "one: $one, two: $two, three: $three"
# output:   'one: 1, two: 2, three: 3'

Can someone explain in what fundamental way do the above two ways of providing input differ from each other (from the point of view of the read command)?

<sub>I do not want to know "how to work around passing input via a pipe", like the linked questions in the comments. I know how to do that (e.g. I can use a herestring!).

My question is, what is the underlying mechanism that makes read behave differently in the two cases?</sub>

The read works, but you need to ask for the values in the same subshell:

echo "1 2 3" | (read -r one two three && echo "one: $one, two: $two, three: $three")

An alternative is

read -r one two three < <( echo "1 2 3")