Because during the time of the musket there were no pump guns neither.

EDIT: It took quite some time between the early days of the industrial revolution and the first semi-automatic guns. Ignoring LOTR for a minute, if in story taking place in the 17th century the question of "why didn't X use a colt .45" is easily answered, because it wasn't invented yet.

If we aren't going to "yes, and..."[1] our way into the actual topic at hand, consider that since they don't have the same standardised inches, even if they did had exactly a Colt .45", they wouldn't call it that, no more then we would call it a Colt .14T (where T is a Gondorian Royal Thumb).

[1] https://en.wikipedia.org/wiki/Yes,_and...

But many parties are sparsely but explicitly described as making and using fantastically advanced marvels of somewhat magical technology in ancient times, some of which are still around like Sauron's Rings.

Probably the most honest reason to not have guns in Middle-Earth is that only stupid orcs, not the elves and dwarves who could easily develop them as school projects, would be interested in such tasteless weapons.

Easily accessible guns require tools that were absent, like screw making machines etc.

There were weapons, powerful weapons at that, but they depended heavily on artisanal production even if certain groups managed to reach manufacturing scale.

Also, Middle Earth is still licking wounds from the great wars with Melkor, which took away most of the advanced regions, and what Gondor and Arnor tried to rebuild got heavily whacked down by wars.

Random subsistence farmers spread across small hamlets do not make for industrial base, whereas Saruman and Sauron could use force to speedrun the development to provide resources for their forces (as both had knowledge of how to do it, even if we only take into account their experiences during wars of Beleriand - meanwhile the wizards other than Saruman stick to their oath of non-interference)

Saruman and Sauron are the only ones who seem interested in industrialization and in mass production for masses of orcs.

Sauron was never bound by the oaths Istari took, and Saruman broke his, and those involved not intervening too much. Most of the time, Gandalf & other Wizards were running around trying to keep things together so that societies could rebuild.

The remaining higher technology groups either lost the capability (Gondor), were destroyed (Arnor), are fighting constant war with Orcs with occasional dragon making problems that prevents building up proper industry (Dwarves), or are depopulated depressed groups that while not hoarding, do not interact with others anymore much and mostly use what remains of their industrial base to facilitate the last days of their race (Elves outside of Mirkwood - those in turn are trying but don't have much to build upon since Moria got overrun and they lost their main trading partner for technology).

The rest is down to subsistence farming levels without base of previous knowledge to rebuild or at least inspire.

I don't think all the Kingdoms of Men would have turned their noses up at a few cartloads of nice Dwarven culverin, regardless of what the Dwarves themselves (or those snooty elves) thought of them.