Because it is 100% secure as long as the key was generated correctly (used a strong random generator) and kept safe.
Well it's as secure as a legacy address can be if they did what you are suggesting. But that doesn't mean they couldn't make it more secure. For example, use an address format that gives more bits of security. 256 vs 160.
Nobody can "get your key" as long as you are protecting it correctly.
I know but ever heard the saying "don't put all your eggs in one basket? In the unlikely event that someone was to find out their single private key they would lose everything. That's why it is smart to diversify your funds. And don't use the same address more than once which they obviously have FAILED to do thus leaking their
public key to the whole world. Now instead of being 160-bit secure, they are down to 128-bit security level.
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Besides if you are incapable of protecting one key, you are not going to be able to protect multiple either.
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Clearly the person or company behind that address has no idea about bitcoin. They are making amateur mistakes.