Another way of looking at it is CDK's main product is an ERP tailored to the automotive industry. These aren't systems with short setup times and there is major risk to trying to replatform you're accounting system. How do you pull of an ERP migration while you're existing one is down? Further, almost everything the dealer does from inventory management to service is integrated to this system of record either directly or via data integrations.
Some of the smaller mom-and-pop stores just use small business accounting systems like quickbooks but those get pretty tedious to maintain with any sizable number of sales or employees per month.