Let's Encrypt encourages automation and does not recommend any kind of manual certificate aquisition flows.
There are many web based flows out there, often with poor security (in particular private key handling). Their existence doesn't mean that Let's Encrypt should promote them, or even offer the same mediocre service.
While this is often asked, this is not going to happen due to the arguments mentioned by Max above. It's simply not a method (manual) Let's Encrypt endorses. Rather the opposite. They'll even go that far that they've removed web-based ACME clients from the ACME client overview page.
And they're publicly visible in Certificate Transparancy logs.
There have been a handful of web based clients in the past.
ISRG / LetsEncrypt officially recommends against Web Based Clients due to concerns over Security and User Experience - most notably the likelihood of missed renewals. Web Based Clients have been actively removed from the LetsEncrypt website. LetsEncrypt actively recommends against third party Web Based Clients, and their comments on the matter apply almost equally to a potential first party Web Based Client.