This has come up before in discussing firearms in Dune and a large part of the problem stems from the source and the Novel itself.
Frank Herbert wrote with the apparent assumption that the risk of shield/Lasgun interaction was so great that the bulk of combat would be melee.
The only actual interaction of them that I think occurs is when Duncan intentionally plants a shield to be triggered by a Lasgun to deal with some Sardaukar during Paul and Jessica’s escape.
I agree that they seem out of place, but hey, got to work with the source material.
As for why ordinary soldiers don’t use guns we had a long discussion here on their use in the Imperium.
My personal head cannon is that well trained troops with Shields and melee weapons will massacre a force armed with guns. Even if they have Shields as well because you are sacrificing skill in melee to train them to use guns as well.
Lower down the pecking order I agree that common forces will have guns, though nothing too impressive as those are the cannon folder troops and you don’t equip them well.
So the art of making guns drops down the order of importance, which impacts how many are made, which affects quality.
A downward spiral as it were.
My players thankfully aren’t focusing on conflict so the issue hasn’t come up.
I would probably avoid them having Lasguns by saying they are too expensive and risky to issue out generally. So you end up with specialist teams that only get used in times of Kanly & War or to put down peasant uprisings with extreme brutality. Such as the Baron allowing Rabban to keep some for use against the Fremen.