microcontroller - Why does AVR code use bit shifting

In AVR programming, register bits are invariably set by left-shifting a 1 to the appropriate bit position - and they're cleared by a ones' complement of the same.

Example: for an ATtiny85, I might set PORTB, b₄ like this:

PORTB |= (1<

or clear it like this:

PORTB &= ~(1<

My question is: Why is it done this way? The simplest code ends up being a mess of bit-shifts. Why are bits defined as bit positions instead of masks.

For instance, the IO header for the ATtiny85 includes this:

#define PORTB   _SFR_IO8(0x18)
#define PB5     5
#define PB4     4
#define PB3     3
#define PB2     2
#define PB1     1
#define PB0     0

To me, it would be much more logical to define the bits as masks instead (like this):

#define PORTB   _SFR_IO8(0x18)
#define PB5     0x20
#define PB4     0x10
#define PB3     0x08
#define PB2     0x04
#define PB1     0x02
#define PB0     0x01

So we could do something like this:

// as bitmasks
PORTB |=  PB5 |  PB3 |  PB0;
PORTB &= ~PB5 & ~PB3 & ~PB0;

to turn bits b₅, b₃, and b₀ on and off, respectively. As opposed to:

// as bit-fields
PORTB |=  (1<PORTB &= ~(1<

The bitmask code reads much more clearly: set bits PB5, PB3, and PB0. Furthermore, it would seem to save operations since the bits no longer need to be shifted.

I thought maybe it was done this way to preserve generality in order to allow porting code from an n-bit AVR to an m-bit (example 8-bit to 32-bit). But this doesn't appear to be the case, since #include resolves to definition files specific to the target microcontroller. Even changing targets from an 8-bit ATtiny to an 8-bit Atmega (where bit definitions change syntactically from PBx to PORTBx, for example), requires code changes.