Why are you selling SEO?

Sure, I understand.
But

Like insurance sales in the U.S. you get paid partly on a continuity, a commission on the monthly bill paid by someone who signed up for insurance.

If you do not continually market your service your residual income does not grow and natural atrition will errode your income.

Don't you think this holds the same for this business?

I do agree with you.

And I've talked about it more in other posts than I have in this one, but if you are good at networking and really help people - they will bring you new clients and grow your business for you.

But as I talked about the CPA association, I mentioned marketing that business... so what you are saying was already implied.