There are countless reasons dealers don't trust the SEM providers. (Or other vendors, for that matter).
I'll just share 10 that my dealer clients most often see.
Starting at the top:
1) Reporting is suspect from many SEM providers.
2) Lack of ability to attach Adwords to Analytics
3) Inability to review Adwords account of vendor
4) Lack of understanding regarding the tech/algorithms making the keyword bids/buys
5) Lack of explanation by vendor as to why certain keywords are chosen
6) Lack of understanding by vendors as to what keywords they honestly should be going for
7) Unwillingness to track clicks down to conversions for some SEM vendors
8) Lack of exclusivity for OEM-approved SEM campaigns cause bidding against local competitor with similar designed/written ads, not helping with awareness
9) Unwillingness to show causation based upon PPC vs web traffic
10) Overcharging for retargeting (even if it is an great buy), DealerKnows sees far too many vendors charging exorbitant flat fees for the service.
This is all in addition to the fact that SEM vendors rarely show up in store to meet and explain all strategies associated with the accounts. Sometimes, something as complicated as a great PPC plan needs to be laid out to dealership ownership face to face. It is also too easy to find one small issue with a dealers' current provider to poke holes in strategy. "Look, if I type in Honda Civics in Saratoga Falls, no ad pops up. Do you have Honda Civics in stock? Well then why aren't you there?!! You need us instead of them" (Those type of BS dealer acquisition tactics hurt the credibility of all.)