Question: > 
> *In mathematics and computer science, a tuple is an ordered list of elements. In set theory, an (ordered) n-tuple is a sequence (or ordered list) of n elements, where n is a positive integer.*
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So, for example, in Python the 2nd item of a tuple would be accessed via `t[1]`.

In Scala, access is only possible via strange names `t._2`.

So the question is, why can't I access data in tuples as Sequence or List if it is by definition? Is there some sort of idea or just yet not inspected?

Answer: 
I think it's for type checking. As delnan says, if you have a tuple `t` and an index `e` (an arbitrary expression), `t(e)` would give the compiler no information about which element is being accessed (or even if it's a valid element for a tuple of that size). When you access elements by field name (`_2` is a valid identifier, it's not special syntax), the compiler knows which field you're accessing and what type it has. Languages like Python don't really have types, so this is not necessary for them.
Apart from the benefits Jean-Philippe Pellet already mentioned this notation is also very common in mathematics (see <http://en.wikipedia.org/wiki/Tuple>). A lot of lecturers append indexes to tuple variables if they want to referring to the elements of a tuple. And the common (LaTeX) notation for writing "with index *n*" (referring to the *n*-th element of the tuple) is `_n`. So I find it actually very intuitive.