Excellent observation! There’s no point in building something without a business case behind it, and it’s technically impossible to build something without engineering. So it’s no surprise that most start-ups have the pair of business and engineering founders, and many businesses still see design as the most optional of those three legs.
What they fail to realise is that design happens anyway. When they treat it as optional, it simply means they do a shitty/accidental job of it.
The same arguments could be made around either of the other two legs, but they don’t need to be made as badly because those two haven’t been ignored to the same degree.