This is considered to be the classical variation of the Open Sicilian. The wikipedia article for this position states "Of all the responses for white, Bg5 is perhaps the best". However, when analyzing with Stockfish, it suggests either Bb5 with a weight of 0.28 in White's favor.
Why is it generally uncommon (unsound?) for white to capture the knight on c6 in open Sicilians?
Here is my take on this, and it is just an opinion. In general, it is better when possible to allow your opponent to initiate a trade because it will usually allow you to end up still controlling that space. This is true in the current situation where, if allowed, Black's knight would take White's, and then White's queen would take.
Is it just because giving away the b-file to black is more dangerous than giving the c-file?
If White does take on c6, and then bxc6 Black actually has a lot of options. It allows for d5 to be played with support from the c file, and overall, it is good if you can manage to take towards the center of the board with your pawns. It also means that, possibly, Black's bishop could end up on a6 which could be problematic in some lines.
But shouldn't white be able to quickly castle on the king side and torment black's a-pawn?
White should be trying to castle and develop in this position. Both of which are achieved with
6. Bb5. This creates an attack, forces Black to defend with his Bishop on d7 (forcing generates tempo), and allows for White to castle all while Black's dark bishop is locked in at f8. Whichever direction Black aims for, you will have at least one move to prepare for it, and that little bit of gained tempo can be the difference in breaking open a position.
Lets just leave Black's a-pawn out of this. It is a tough cookie and will be around for a while.
Goal In My Opinion
[fen "r2qkb1r/pp1b1ppp/2nppn2/1B6/3NP3/2N5/PPP2PPP/R1BQ1RK1 w kq - 0 8"]