If you are a home user, Windows Defender is fine for single computer protection. However, it does not provide the level of management needed in business environments.
Businesses have more to worry about
Businesses that rely on a large volume of information must ensure the security and privacy of their users. They must follow certain rules and standards that domestic users do not need to worry about. This is because home users only have to worry about their own data and not others'.
Its one solution to security threats
A single antivirus is not enough to protect against all security threats. You need to have a multilayered approach because no one system is perfect. You need to build systems like a castle to defend against cyber threats.
Social Engineering is more dangerous
I consider social engineering the number one threat than anything else. Systems are hard to compromise. It also takes numerous times to penetrate multiple systems. Not like in the movies. Humans are easier to con and the reward is faster. It takes a 10 minute phone call from a scammer to get you to pay them vs hacking into a company's computer system. Which is why I think that it is more important to educate people on how they can avoid falling victim of these crimes.