I'm not sure that qualifying these links of "very hard" to distinguish is the right term, at least not on a general basis.
However, where I agree with you is that depending on the screen settings and potential visual limitations affecting the reader, these links may indeed be hard to distinguish from normal text.
That's the reason why often visual indications such as links rely on two different criteria, traditionally links are shown:
- Using a different text color,
This also allow the reader to detect successive different links, which is just currently impossible on StackExchange without hovering the links.
However, underlined text within a paragraph may also slightly reduce readability and comfort (that's why Wikipedia for instance choose to underline them only while hovering, as a right balance), and a poster on UX.se also suggested the following:
I'm pretty sure that this is an intentional part of the Stack Exchange
Answers are supposed to be standalone, because external sites may go
offline, or rename/take down the page linked to. If they primary
content of the answer was a link to another site, then that answer is
The links are not underlined, so as not to distract from the answer,
since they're supposed to only go to supplemental material. Similar
reasoning exists for using colors that have less contrast to the text
as opposed to the general web.
So, without being into StackExchange little secrets, chances are this may be intentional and that arguments privileging text readability and unity over links parsing easiness simply won.
You may also find relevant information in the posts below from UX.se: