Most platforms have an owner - for example Android is owned by Google. On these platforms the owner usually provides a store with payments, ads etc.
The web has no owner. In other ways this is a big strength, but on this point, it means there isn't one organisation who provides a store with payments, ads etc. Instead there are dozens and dozens of different organisations and services all vying to be "the app store of the web" or some other equivalent, but in practice it's a highly fragmented market with nobody in any dominant position.
We only have a few developers, and integrating services can be a huge amount of work and then have high on-going maintenance costs. For example integrating Facebook Instant Games was a big project taking months, still doesn't cover the entire available API, and occasionally requires us to drop everything and go and quickly fix something that got broken because they changed something about their service.
Given our very limited resources and the vast array of services out there, it's simply not feasible for us to cover even a fraction of them. This is exactly the purpose of the Addon SDK: it allows third-party developers to come along and integrate Construct with their services without us even having to be involved.
There are some addons out there already, such as for Lagged.com. In other cases you can ask the organisation to write their own addon with our SDK, or if you have the capability, write an addon yourself, or hire someone else to do it.