I assume by "American comics" the meaning is like... American serialised print comics, like Marvel, DC, Dark Horse etc? Because there are plenty of webcomics by American creators, and have been since pretty much the advent of webcomics.
The thing is... they are online, and have been for ages. Marvel have been trying to do subscription model access to digital versions of their new releases and back catalogue, not dissimilar at all from the shounen jump app, for close to 20 years now, and Comixology was doing so well for a while it got bought by Amazon who...er...seem to be trying to kill it and fold it into Kindle and stuff.
It's not a lack of trying so much as that that style of American comics aren't well-suited to the digital format and don't feel relevant or fashionable any more. The pages are large, meaning that they don't read well on the most common device people use to read comics now; a phone. The art has become more and more highly detailed over the years so that now it's impossible to create pages at the speed or with the small team needed to make a profitable comic, but they don't want to make the art simpler or go back to flat colours like old comics or black and white because it'd look like a step down in quality, so they're stuck. The characters don't resonate with younger readers, but they can't reinvent them like the movies can because they're scared to piss off the die-hard fans who spend the most money right now, and also their writers are all getting old and out of touch, so when they do try to reinvent characters or add new ones, it's often cringe to modern young people.
They're stuck between a rock and a hard place. The people spending the most money on American print comics don't want them to change, but they need to change to grow the audience because not doing so will cause them to die, so they need to simultaneously keep making the print comics for their old audience, while making comics designed for web and for a modern young audience in format, style and tone at the same time, so you have DC making those very detailed, gritty Batman comics for print...while also making a batman webtoon that's long-scroll format with cute manhwa-influenced art and a focus on younger characters and a slice of life tone.