In Chrome (and most other browsers from what I can tell) the font used has the issue that the glyph used for I (uppercase i) is identical to the one used for l (lowercase L)
This is something I have always wondered. Particularly when it comes to the URL-field, as this is literally inviting phishing attack. But overall I find it rather annoying that it seems to apply to whatever is the default-font on webpages as well (and if not, the majority must explicitly be changing it to a font with this issue. In which case, the "why?" applies to that question too).
PS: While it would be nice to know how to at least change it for myself (which is an easy-to-google question, and for chrome involves chrome://settings/fonts), this question is about the why, not how to fix. Besides, I would much rather see chrome (and other webbrowsers) fix this default for everyone, as the current state of things seems negligent in my mind.