Because successful remote arrangement at a company with an office depends on:
1. A certain culture shift (recording presentations or forwarding decks to remote employees).
2. Relevant support tools and processes (dial-in numbers for every meeting, screenshare software, etc).
3. The right attitude (discipline) from the remote employee. Not everyone can be productive at home. Not all roles are a natural fit for a remote arrangements. More junior employees typically need in-person guidance and mentorship.
It's also harder to evaluate someone's performance in the first months on the job. Many smart startups that are open to remote arrangements will ask a new hire to work at least the first month in the office, before switching to a remote arrangement. It's fairly common to let people with seniority (2+ years with company) to switch to remote arrangements because they have proven themselves as top performers.
In other words, if it's not a general/common practice at a company, introducing exceptions causes some overhead and requires additional effort. I would speculate that it might be easier for a brand new startup to just adopt a distributed team approach, then for established company to introduce remote arrangements.
I personally hope more startups consider distributed team setup and think that perhaps the trend has already began.
Some startups do. Especially after being unsuccessful in finding a candidate locally for some time.
Managing remote team members has its own set of challenges. Some of which include:
Those are the main reasons why most startups tend not to first look at remote candidates.
I feel the benefits out-weigh the cons though:
Things you should look for when hiring remote people (in order of importance):