Why is Scala's syntax for tuples so unusual?

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.

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?

Scala knows the arity of the tuples and is thus able to provide accessors like _1, _2, etc., and produce a compile-time error if you select _3 on a pair, for instance. Moreover, the type of those fields is exactly what the type used as parameter for Tuple (e.g. _3 on a Tuple3[Int, Double, Float] will return a Float).

If you want to access the nth element, you can write tuple.productElement(n), but the return type of this can only be Any, so you lose the type information.

I believe the following excerpt from "Programming in Scala: A Comprehensive Step-by-Step Guide" (Martin Odersky, Lex Spoon and Bill Venners) directly addresses both of your questions:

Accessing the elements of a tuple

You may be wondering why you can't access the elements of a tuple like the elements of a list, for example, with "pair(0)". The reason is that a list's apply method always returns the same type, but each element of a tuple may be a different type: _1 can have one result type, _2 another, and so on. These _N numbers are one-based, instead of zero-based, because starting with 1 is a tradition set by other languages with statically typed tuples, such as Haskell and ML.

Scala tuples get very little preferential treatment as far as the language syntax is concerned, apart from expressions '(' a1, ..., an ')' being treated by the compiler as an alias for scala.Tuplen(a1, ..., an) class instantiation. Otherwise tuples do behave as any other Scala objects, in fact they are written in Scala as case classes that range from Tuple2 to Tuple22. Tuple2 and Tuple3 are also known under the aliases of Pair and Triple respectively:

 val a = Pair   (1,"two")      // same as Tuple2 (1,"two") or (1,"two") 
 val b = Triple (1,"two",3.0)  // same as Tuple3 (1,"two",3.0) or (1,"two",3.0)