It's not just that they can't afford to measure their return, but that often you can't afford to spend money on marketing that doesn't have a direct return.
When I started consulting, I spent some money on marketing, but my big constraint was that if a given channel didn't at least break even on direct conversions, there was only so much money and time I could afford to spend on that channel for conversions down the road by increasing my brand: It's not just total lifetime returns from the advertising that matters, but how the timing of those returns affect cash flow.
If the time it takes to break even is too long, you might be bankrupt first, even if the potential lifetime return is fantastic.
Sometimes it costs a lot to not have the time and budgets to take a longer term view.
But small companies don't even know how to measure such returns, or can't afford to measure their return beyond sales and analytics data. That's why it will always be a gamble for them, specially like you said, it's not the best place to start.