Why? I've been running services on single instances for 13 years and it's been ok. Not everything has to be webscale.
It's not about being webscale it's about being professional.
It's not professionalism to increase the cost of your service regardless of your needs. Maybe 99.99% is good enough for the OP's price point, and if I want five nines I'll pay for Zendesk.
In this sort of situation, using multiple servers is likely to reduce reliability. Needless complexity is not a good thing. Making a system distributed has a real cost.