It's better to be proactive than being reactive. Take preventive measures that fit your threat model.

I do, for three reasons:

1. Hackers. It's unlikely that anyone would hack my webcam, but there's always a chance. Maybe I'm paranoid, idk.
2. Hardware exploits. Three of my laptops are too old for me to update the firmware with fwupd, so I cover the webcams in case there's some critical hardware-level vulnerability which could be exploited; or in case one of the three-letter agencies are in there.
3. Consequences. Despite the incredibly low chances of anything happening whatsoever, the possible consequences are too bad for me to want to risk it.

I'm paranoid, aren't I...

Is it unnecessary to cover one's webcam on Linux?

No. Please cover your webcam.

how vulnerable is the device to having its webcam exploited?

Every bit as much as Windows minus their proprietary spyware.

How vulnerable is a Linux device if you don’t take extra precautions like firewalls.

Depends on what links you like to click.

what makes Windows so much more vulnerable?

Fewer eyes on the source code. Effort to reward ratio, the 80-20 rule. 20% of the effort nets your 80% of the reward. Literally. Develop exploits for one platform, target 80% of average computer users. Or write exploits for hundreds of different distros for *checks notes* ... 4%. Unless you like servers. There there's a coin toss. 50% linux, 50% Windows.

Keep yourself safe, there's malware for Gnu-Linux too. Install your patches when you can. Remove software you don't use. Practice good cyber hygiene.

The thing about exploits is that they can exist without there being a wider knowledge of them; that's the nature of the beast. So I play it safe.

Plus, for 5 cents' worth of masking tape you've solved the problem, so why bother even thinking about it.

Most malware is writen for Windows, but Linux malware exists, and has been found in the wild. The most common infection method, like with windows is running a trojan, theoretically things like browser exploits could also be used, but these are unlikely unless you are a high profile target like a head of state or CEO.

I would personaly be much more worried about someone evesdropping through my microphone. All they would get from my camera is my face and some glances of the room my computer is in, but my microphone would reveal all sorts of private conversations.

I never really understood the obsession to cover cameras. While yes you don't want someone to hack it and yes I do cover mine. What are they going to do? Watch you fap? I really don't care. I personally am much more concerned with the mic. I would hate for someone to record me saying something well not so pleasant. You can use your imagination. I rarely see people mention the mic. I see it mentioned a few times here but not much. I much rather have a kill switch for my mic.