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You can afford them if you don't have project managers or sales guys begging for their demoable results of new shiny thing and promising it to others too early and too often.

My companies first 5 hires were junior developers. This cost a fair packets (because there were 5 of them!) and led to pretty low quality software (because they were all junior and didn't know what they were doing).

They could have shipped higher quality software quicker by hiring 2 senior developers for similar cost.

Startups bias toward hiring some less experienced developers by their nature, so they don't always know how to build quickly and solidly. The up front cost is either learning skills of abstraction from scratch, or hiring people who have.