Why do companies lie about uptime?

Why do companies lie about their uptime?

I'm not trying to call out Sentry.io specifically here, I'm just using them as the example because it happened today. In fact, in general I'm a happy customer of theirs.

I'm trying to understand the motivations behind companies that aren't open about their uptime (or other issues, such as data breaches etc).

Why is it not encouraged/rewarded (or perceived as such) to be up front and open about these things? It seems that amongst the HN crowd it has been valued, and folks are appreciative and supportive of these things (vocally and with their wallets) - so why is it not more common practice?