One thing though, is many, perhaps most (not all) churches that believe in YEC also believe in Dispensationalist eschatology. This is virtually every fundamentalist church and many churches that are Evangelical (in the American sense of the word). It is a pretty good bet that if you picked a non-denominational Evangelical church at random and asked, you would find that it teaches dispensationalism.
What does this have to do with YEC? The usual dispensationalist timeline, which many such churches believe in, puts the earth from creation until the end in a span of exactly 7,000 years. This belief system began around 200 years ago and many people place their hope in this supposedly Biblical timeline. This timeline is treated like a creed. These churches eagerly welcomed the modern YEC movement (YEC gives “scientific” validation to their timeline). I think this is why it is extra hard for some to entertain even the idea of an old earth.