Probably because it is more work in that environment. To accept payments for an ad-free version you need to create and maintain some sort of payment processing, and of course user authentication & session management. With iOS apps a lot of this is baked in.
Also it is much easier (for a non-technical user at least) to just skip the ads on the web using browser plug-ins, than it is to get around them in an app, so to an extent this extra work may be wasted and the number of people who take up the "pay for no ads" option might not be enough to warrant the work that goes into maintaining it.
There are some sites that offer the "ads or pay" option so it presumably works out worthwhile for some sites, but I've not seen it often.
Also it is much easier (for a non-technical user at least) to just skip the ads on the web using browser plug-ins, than it is to get around them in an app, so to an extent this extra work may be wasted and the number of people who take up the "pay for no ads" option might not be enough to warrant the work that goes into maintaining it.
There are some sites that offer the "ads or pay" option so it presumably works out worthwhile for some sites, but I've not seen it often.