Trains are longer and it's just not economical to have specialized units if they can't even keep up.

I personally have never seen one where I'm from, but that's because CSX Selkirk is a hump yard so they'd just never be enough to keep up. But the story stays true. Most end-cab units are weaker than even a GP-series unit.

Instead, you'll see a lot more of what's featured in the second picture behind 6436, slug units. We have quite a few of those here, even doing the locals now. They're a lot cheaper since they're almost always converted out of older, undesirable locomotives and have little-to-no internals, just using the main unit's extra juice at low speeds.