I am a little late to the party, but want to address one topic that I didn't see mentioned below. One answer to your question is "volume", with a related cause

I am a little late to the party, but want to address one topic that I didn't see mentioned below. One answer to your question is "volume", with a related cause of personal incentives. You're getting those clicks for free and buying ads on branded terms is just cannibalizing your organic results. Now, instead of $500 per day, you want to scale your campaign to $5,000 per day. How do you do that? Your return will be less efficient as you seek volume, but you need to spend the $5,000 as effectively as you can. You (or the agency) look better / get paid more when they're able to spend with a better return. It's probably not your department, or your KPI, what the organic SEO is like. So if you cannibalize some conversions from organic in the SEM attempt to spend the budget and raise conversions, you're probably happy, because you're hitting KPIs that matter to you. It would take a wise leader to recognize what is happening. Even then, they might decide that although the efficiency of the overall SEO/SEM spend is lowered, it provides more volume. If your company is focused on growth, they may prefer volume over efficiency. It's rational, once you try to see the larger context in which these decisions are made w/in a company. (It also has it's drawbacks)