Code Fence Syntax -- why not exactly 3 backticks?

From the code fence section of spec:

A code fence is a sequence of at least three consecutive backtick characters (`) or tildes (~). (Tildes and backticks cannot be mixed.)

Is there any reason not to define the length of the code fence to be exactly 3 backtick or tilde characters? This would make parsing a lot easier in certain pattern matched languages.

I think it’s so that you can describe code fences inside of a longer code fence.

For example (note, this is an indented block, because I can’t use a longer fence here (yet)):

Format your code like this:

code here