I'm aware that there are some other brands but they all have issues which not necessarily place them in competitive place vs Samsung, some examples: Motorola (horrible updates policy, even worse than Samsung) Sony (small selection of models, pricey, no idea about updates), Pixel (not available in some countries, bad battery life, quality issues, price)
If you seek out near-stock phones they really do exist.
They were also basically the only game in town for high end android tablets with OLED displays for years. I'm really happy with the hardware and build quality of my Samsung tablet, it's the software that's a pain in the ass, and I'd have no problem recommending their devices to people if LineageOS support were more reliably available.
Tl;dr- they're basically the go-to consumer Android mfgr, people are used to buying from them and their devices are generally a safe choice.