Mostly because battery life tends to suck. I can still play games like Minecraft and most indie games on my iGPU (esp if you go AMD), so it’s not like there aren’t options, I just won’t be playing larger AAA games on it.
I haven’t shopped too much either, but in the past, gaming laptops were significantly more expensive and often had quality issues vs business-style laptops. I quickly checked prices online, and “inexpensive” gaming laptops seem to start around $1k for a 4060 tier GPU. That’s the price of a decent business laptop (should last 5+ years) and a Steam Deck. You’ll probably want to upgrade the Steam Deck way before the business laptop, so it should help with costs longer term (and by then you’ll probably want a desktop PC anyway).
Anyway, that’s my take. I personally don’t see a point to gaming laptops, for the game money, I’d rather have two dedicated devices.