Lately it feels like “buy” buttons, which add the ability to shop directly from webpages and apps, are the hottest thing to hit the Web. Facebook has plans to add buy buttons to its social network. Twitter is already experimenting with them. After years of anticipation, Pinterest last month launched its own buy buttons. This morning, Google announced that buy buttons will appear in its mobile search results.

This morning at Fortune’s Brainstorm Tech conference in Aspen, the co-founders of digital payments startup Stripe explained what’s driving the buy button phenomenon. The short answer? Mobile.

Because mobile device use is dominated by apps, not websites, it’s difficult to navigate like you can between various websites on a desktop computer, said Stripe co-founder and CEO Patrick Collision. Multi-tasking comes naturally on desktop computers, but not on tiny mobile screen where everything moves slower and doesn’t work that well.

“Apps are trying to flatten the funnel so everything is right there at the point of discovery,” he said. In other words, you’re more likely to shop directly inside the app you’re already using than if you’re sent to a new site or different app. Stripe is helping to power many buy buttons with its payments software.

John Collision, the brother of Patrick and co-founder of Stripe, compared the buy button phenomenon to the way media startups like Facebook FB and Snapchat are pulling more content and actions into their own apps. Snapchat, with its Discover tab, now places video content directly inside Snapchat, rather than send users elsewhere. Likewise, Facebook’s mobile ads have been successful because they now include a button that allows users to install apps directly from the ad. “The point of purchase is going to move from being on the merchant’s site to being directly into the app,” he said.

That’s particularly important as consumers shift their web-browsing habits to mobile, where conversion rates are lower than on desktop computers.

Will consumers get sick of buy buttons? Patrick is skeptical. Shoppers will “think, ‘Oh awesome, I don’t have to go to this site… I can perform the action in a single step,'” he said. “The apps that don’t have this are going to seem broken by comparison.”