Hi. I can see what you are talking about. Based on your photos, it looks like your tree may be experiencing some branch dieback. That can result in a thinning canopy, with large branches (that are mostly dead) having only a few leaves remaining on terminal branches. The fact that you have been seeing this for several years suggests that this is a progressive issue rather than something with a sudden onset. There are a variety of possible causes for canopy dieback in maples, and this page from the U. of Minnesota has a nice list of them with descriptions of symptoms and causes. This page from the U. of Illinois is also good, as it lists the issues in approximate order of frequency. I can’t distinguish among the possible causes without additional information and photos of the trunk and roots, (and even then it may not be possible) but reading through the options may allow you to narrow down the possibilities. That page from the U. of Illinois says “The exact cause or causes of decline and dieback needs to be identified so that corrective steps may be taken. Accurate diagnosis is often difficult however, especially on older trees. Usually an on-site examination of the diseased tree is required to assess the influence of the environment and to inspect for foliage, branch, trunk, and root problems.” I suggest you pay attention during the growing season to see if you can identify particular branches with leaves that turn yellow and die, as the manner in which that happens can provide clues as to the cause. It could also be due to a combination of factors. For example, it could partly be due to winter injury (an environmental stress) in combination with stem girdling roots (which are common in maples and can take many years to develop into problems). Unfortunately, for most of these issues, there are few options for treatment other than just being conscientious of regular tree maintenance, such as pruning out any dead branches and providing additional water during periods of drought.