A big part of it is that if you're in a very competitive realm, where most of the startups you hear about are working, then every day counts. If you can spend $1M to develop a product in a year or $2M to develop it in 6 months, that extra million gives you a 6 month head start in sales, revenue growth, and publicitity. Depending on the numbers involved, that frugality could cost huge amounts of money overall.

Note that you don't hear so much about the many, many startups doing slow growth things in less glamorous fields. I know a few companies making agricultural products for small farmers. Yes, frugality makes perfect sense for them. They're not going to have a hockey stick growth curve where they go from $0 to $10M to $1B over the course of 2 years. Their revenue graph will look more like a traditional manufacturer. They're doing things the way you describe, but they're not all over tech and non-tech news sites.