Lack of manager ability to measure productivity based on anything else. Between POs, BAs, business representatives, and IT managers there are a lot of people who don't seem to be able to grasp why two relatively simple to describe tasks can require such vast differences in time. So it becomes hard for them to judge if their developer is wasting their time or not (and let's not even begin on overall lack of trust to begin with), so the only fall back measure is if they have their butt in the seat and appear to be working. And even if you are lucky enough that everyone you directly deal with either understands the basics on why sizing varies so greatly or able to accept it without question, you still have to deal with middle managers walking the floor who measure productivity the same as they would in a factory. And if you are lucky enough to not even have this problems, you still have to deal with cultural norms set in party by all the jobs which aren't as lucky.
As people are noting on this thread, startups are their own thing. During my startup days, we were always the next sale or funding round away from going belly up. We pulled late hours to get in the latest features for our president to "sell" to investors.
Had a lowball offer from a company once - had some friends working there. I could see from some earlier code that some really basic web knowledge was being overlooked. I won't go in to details, but... this was something I'd pointed out to one of the managers there (met at a local meetup). Fixing it then would have taken about 2-3 hours, scanning all the code to ensure it was in sync. Instead, the problem spread as they grew, and it's harder to fix when there are dozens of staff and thousands of files in use vs 5 staff and a dozen files. It became a 'this has to be fixed in order to close mega-deal X integration with company Y', and it took multiple devs several weeks of full-on effort to rectify everything.
How do you quantify that? "Please, take my advice, doing this 4 hours of work now now will save you >$50k and weeks of opportunity cost in 4 years". Generally people don't seem to care.