Why aren't guitar and bass speakers flush mounted on baffle

[RDK]

First off. What are the pros and cons of flush mounting speakers on the baffle and why aren't guitar and bass cabs built like this.

[Sydney]

The output of Guitar and Basses is in the low to mid octaves and not the top octaves; Guitar and Bass speakers are not designed for the top ( HF ) range where flush mounting is of greater significance to reduce diffraction of the top octaves.
Tweeters are a different scenario as HF wavelengths are much shorter.

Guitar cabs and speakers often trade and compromise on issues that are important to HiFi scenarios; because they are an integral part of the sound creation - as opposed to sound reproduction without color.

Drop-in mounting driver mounting is easy to manufacture and facilitates replacing drivers from the outside.
And there would be additional fabrication cost to flush mounting; besides that drivers in guitar and bass cabs are almost always on a recessed baffle ( often behind a grill ) for protection against "road accidents"

[billfitzmaurice]

Flush mounting does nothing for the woofer, it's intended to reduce diffraction of the midrange and tweeter wave fronts off the woofer frames. Electric bass cabs in general are poorly engineered, if at all, so the last thing their manufacturers would do is increase build cost by flush mounting the woofers.

[Pete Schumacher ®]

4 word answer.

It does not matter.