Amphibs have a higher burden of pilot responsibility, kinda like a twin. And, they have greater operational capability. If you need the capability, you manage the added complexity. Though more complex and expensive, amphibs will allow you to pick up your passengers at the airport, and take them directly to the lake. If the lake is not served from shore at all, this is pretty important. Amphibs will allow you to fuel at an airport, rather than having to get avgas to a lake, and fuel a floating aircraft. You have the choice to tie the amphib down into an airport tiedown spot, or even hangar it, and not worry about it alone at the lake, or sinking there un noticed. And, amphibs can be operated into an airport at night, float flying is day only. I have two amphibs, and manage them with discipline. The afore mentioned prelanding checks are made and spoken before every landing, if you miss the check, you risk the plane - so don't miss the check. If, on short final, you don't recall doing the check, it's an automatic go around. I don't like the advisory systems, they give a false sens of security. They are not absolute, and if they fail, it's a benign failure, which likely goes un noticed. It's best handled with simple pilot discipline. The same discipline is needed for wheel ski operations, for which I have never heard of an advisory system, or other means of warning. If you land a taildragger wheel ski plane skis down on a dry runway, you're going to wreck the plane..
A floatplane operator does not want an amphib unless their operation warrants it, they're more expensive, higher risk, and have a lesser useful load. So if they have one, it's because they have the operational need.