Why do people think indicators are optional?

I was at a busy roundabout this morning and I swear, about half the drivers were just treating their indicators like they cost extra to use. There’s nothing more frustrating than waiting for a gap, only for someone to turn off at the last second without a signal. You’re just sat there like a lemon when you could’ve easily pulled out if you’d known their plan.

Is it just laziness, or are people genuinely forgetting how to drive the second they leave their driveway? I feel like I’m playing a guessing game every time I get behind the wheel lately. Surely it’s not that hard to flick a stalk and let the rest of us know where you're actually going!