Is that supposed to be an argument for having them as variables? If Bash had a function named random rather than a variable then you could do this

echo "'$(random)' is a random number"
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If the extra ()s are too much of a burden then the logical conclusion is that everything should be a variable just to make it easy to do in string interpolation.

The principle of least surprise means that you shouldn't make something look like a variable if it's not going to behave like one. For example

MINUTES=5
echo $MINUTES
# 5
SECONDS=5
echo $SECONDS
# 1276
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How can that be a good thing? There's not even a warning.

Also, having variables that aren't leads to run time errors that could have been caught at compile time. If random was a function, attempting to assign to it would be an error (at runtime in bash but at compile time in perl).