Co-founding a company and building a magical product is like making love, having a child, and raising it. It's hard to do this successfully from a distance.
It's hard because, in the early days, startups are all about learning and moving fast. Working remotely, especially from a distance, reduces your ability to communicate, share information, and collaborate. You can't turn around in your chair and say what if we did this, you can't vent about what's not working on the way to lunch. You can't see the frustration, confusion, or excitement in each other, which is important information that fuels additional conversations and leads to better decisions. In other words, working remotely slows you down when you should be trying to figure out ways to move faster.
Maybe you work remotely with your co-founder right now, and you're doubting this. Well, ask yourself: If there were two versions of your team, one fully remote and one in-person, which would you bet on? If you've worked together in person and remotely, which time did the startup move faster?
I'm not saying that all remote work is bad. Working remotely might be better for you if you're an individual contributor. You have a specific task where you're heads-down researching, designing, coding, etc. Working remotely might be better for you because you don't have distractions from other colleagues or the bureaucracy of the company.
If you're trying to create something new with partners, whether as parents, co-founders, a small team within a big company, or a group of executives, you should do it in person if you take what you're creating seriously.