Why would a browser(be designed to) care about this?

Because people download viruses from the internet all the time? "Common sense antivirus" might work fine if you're technically inclined, but that's not the case for everyone.

	▲	mrob 2 days ago \| parent [-]
		The growing prevalence of so-called "supply-chain attacks" (a bad name because it implies a commercial relationship that doesn't usually exist) shows that "common sense antivirus" isn't working so well even among the technically inclined.

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rcakebread 2 days ago | parent | prev | next [-]

Because Google owns Youtube.

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reactordev 2 days ago | parent | prev | next [-]

To protect the normies from harmful malware… not on their approved vendor list.

	▲	exe34 2 days ago \| parent [-]
		it's to protect shareholder value.

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thebeardredis 2 days ago | parent | prev | next [-]

Because Google does no evol.

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g947o 2 days ago | parent | prev [-]

You could also ask why Android care about banning side loading to "prevent scams and spyware", and I honestly don't have an answer at all.