Because you use it all the time at least one piece of software is broken for you at all times.
I stopped using Facebook after my freshman year of college, but recently got pulled back in by a Facebook group. As a result I now have the pleasure of enjoying a 10+ second loading phase every time I open the homepage.
Recently, I tried to buy a CODE mechanical keyboard on the wasdkeyboards.com website. But every time I submitted my order it failed. I tried different browsers. I had to look into the console to find out that a http request was failing to find a paypal advertising domain that my PiHole blocks on the network. To buy my keyboard I had to tether wifi from my smartphone. A non-technical user wouldn’t have been able to find out why the order failed because there was no error message. There was a spinning symbol that just disappeared after a while without a message to the user.
Everyone uses software all the time now. We have smartphones, smart TVs, smart refrigerators and smart homes. If you use 100 programs a day, 99% uptime means one program is down for every person. If every application manages 99.9% uptime, one out of a hundred people is experiencing software brokenness everyday.
Then realize that billions of people have smartphones now.
99.99% * 1,000,000,000 = 100,000.
If your software has a billion users and works 99.99% of the time, its down for 100,000 people all the time.