Many SF tech startups overtly believe that actually planning your code before you write it is just, well, not very "agile" -- that's old school, that's waterfall, proven to be deficient, that's not how we do things in the new world. Planning an architecture before coding it?? Why, you may as well be wearing a pocket protector and hating on women and minorities from some massive IBM cubicle farm in the suburbs while chainsmoking and punching your code into paper cards.
In the new world, you break things and move fast -- when you're assigned a massive new project, you just sit down and start typing. When your unplanned, non-architected code turns out to have serious structural issues that make it unextensible and unmaintainable, why, you just layer another level of crap code on top of that to plaster over the cracks, and another on top of that, and so on.
I wonder how much of this has to do with the churn-and-burn VC culture, where they fund hundreds of startups with just enough money to build an MVP so they can "test for market fit," then burn 99 of them and move on to the next batch.