As the title above, why aren't 1.e4 g6 2.d4 Bh6, and 1.e4 b6 2.d4 Ba6 (or eventually beginning with 1.d4) for Black not even considered by theory? Isn't the eventual early trade-off of White's strong Bc1 and Bf1 for Black's Bishop an asset for Black? What is the best reply for White vs such irregular defences? Should he trade the Bishops or avoid that? (i.e. with f2-f4 and c2-c4)?
White should trade bishops and use the unprotected knight to gain some extra tempo and gain a massive lead in development, e.g.
[FEN ""] 1. e4 g6 2. d4 Bh6 3. Bxh6 Nxh6 4. Qd2 Ng8 5. Nc3
4... Ng4 is best met by 5. h3 Nf6 (forced) 6. e5 Nd5 7. c4 Nb6 8. Qh6 and Black has problems completing his development, though there is no forced win at the moment.
I haven't analyzed the b6/Ba6 lines but I guess a similar concept holds.
I would argue that both of these lines are incorrect on principle. g6 weakens the dark squares on the kingside. In such positions, typically one wants to keep the dark squared bishop on the board to help defend the dark squares. Playing Bh6 lets white trade the dark squared bishops for nothing.
Moreover, the reason for playing a move like g6 is usually to put the bishop on the long diagonal where it can be very strong. If you just trade it off instead, black has gained nothing by the tempo that was lost when g6 was played.
The same applies to the other line.
Opening moves that don't have remotely reasonable ideas don't have a place in theory.
Your only assertion seems to be that Black is eliminating "strong Bishops". Why are they strong? They haven't even left home yet.
A) White trades bishops: Black ends up with a silly looking undefended knight on the rim coupled with weakened squares.
B) White doesn't trade bishops: White can just keep developing, it doesn't really matter how.
Either way White ends up with a large development lead and the center.
The assumption that White's bishops are much stronger than Black's is faulty. After 1...g6, Black has weakened his dark squares and requires a bishop to "fill in the gap". Playing 2...Bh6 betrays this duty, and to make it worse Black's knight will have to go to the unattractive h6-square to recapture. The same logic is applied to 1...b6, 2...Ba6.
Against the finachetto White often spends a few moves to actually initiate a bishop exchange (e.g., Be3, Qd2, Bh6).