If I was selling a 10000 used truck id clean it myself so it would sell faster.
Exactly. YOU would clean YOUR vehicle. But you would not give someone else 20% to clean it for you. Now put yourself in their shoes. It's not the RE agents house. And who does the RE agent want to pay for the cleaning? The owner.
I don't think it's that they don't want it clean, it's that situationally it's not something they have control over so they just accept it. And they have accepted it so long, it just "is" (see Ken's comment). Test your ehory; Run a $29.95 special for RE agents. I think you will find that a lot of them care. They just can't (mentally or actually) see it providing enough value to justify a 20% hit for it.
Now, if you want to seriously play in this market/space, find out their avg time on the market and their cost per day on the market. Work out a plan where they pay you so much for every day under the average and nothing if it sits longer. For example;
Assume
------------
$300 wash job.
Avg days on market 90.
Avg agent cost per day, $10.00. (This is a WAG)
Plan:
We wash for nothing up front, you pay me $10.00 for every day under 90.
Benefits to agent:
Their total cost is not increased by hiring you.
The have no risk.
Houses sells faster.
Benefits to you:
If house sells in 60 days or under, you make premium $$$
I think what you will find (and your agents are telling you indirectly) is that their carry costs are low enough that there is rarely a scenario in which they stand to profit (or break even) by adding your service. Even if their costs as I assumed (high), your services must make a 33% (or better) reduction in the sales cycle to benefit you. And while a wash job will help, that much of an impact is unlikely unless the house looked like s**t before.
When it's all said and done, everyone is trying to make money. If you can build a case and show where your service creates a net gain, you will have TONS of RE business coming your way.