For what reasons don't lung and asthma doctors take lung xrays when you go to them?

No advantage: In the majority of cases, an asthmatic's lungs are normal between attacks. Xrays would not show anything. If your physician suspects your attacks may be due to obstructions or something else, he/she may want to check with xrays but this is not the usual case.

Need vs expectation: No test should ever be done just to do it. Any procedure has some risk & all require a cost in personnel or lab time. A physician learns over time which chests need an image taken and which do not. The history and physical exam are the best screening tool to decide when or if studies are needed. A doc or clinic that does a full panel of tests & x-rays every time you visit is a billing mill, avoid.

Agree : Once is fine and if new symptoms then repeat may be appropriate. Sometimes may want CT Scan or other sophistication of testing. Brewing tests with an without medications may be indicated.