Programming

The answer to this question is the same as for all documentation:

1. Because it's boring to write documentation
2. What is obvious and what needs to be explained is often not apparent for the person creating it
3. When time is limited then documentation is the first thing to be reduced
4. Not enough value is put on good documentation and the associated skill

But in the spirit of FOSS documentation is something that is actually an excellent thing for a person new the project to start contributing to. You need to understand the code anyway to be able to help with bugfixes / feature creation so might as well build reputation by improving on existing documentation by adding clarification and comments and wiki entries that would've aided in making it quicker for you to grasp something.

There's no point spending twice the time making something that corresponds 1:1 to code - unless the code is truly horrific, and then you've got bigger issues. One generally assumes people involved with a project will know or learn relevant languages.

It's more commonly used for things that are less obvious from the source itself - like "what's sent between the client and server in this handshake" or "what is the overall architecture of this part of the code"

Flowcharts may be a bit more common when studying algorithms in general

Edit; from your other comments, you seem to be talking about protocols, not algorithms. An algorithm is a set of steps to do something. A protocol is a description of how different systems interact such as in a chat application.

"algorithm" is a little too fine grained for what you describe. "software design patterns" is probably what you are looking for.

unfortunately, not all projects call out the patterns they employ.

Considering that reading source code can take a long time

You'll get faster over time, until reading code is faster than reading documentation, as code will always represent what's truly happening, while docs are frequently outdated.

In a language the user isn't familiar with

If you're not that familiar with the language, it's likely you won't be contributing to the project. Open source projects usually to have quite limited resources, so they tend to optimize docs and dev UX for people who are likely to contribute.

Most software is poorly documented. That applies to FOSS as much as to commercial projects.

Being able to document the exact algorithms used to solve a problem is sort of a 'Holy Grail' in programming. Most programmers aren't afforded the time to document to that level. This would require refining the code down to a single cohesive story that describes the functionality of the implementation. It's great when you can spend the time on it, but the problem is that the code is actually "working" long before that point. Most employers want to stop paying for code after it starts working.

At the end of the day it's a lot of effort.

People are already giving up their free time to write code.

Very few companies even have this.

You are really looking for architecture diagrams. These are extremely rare in most projects, open source or otherwise.

The reason you don’t see a lot of documentation on algorithms used or architecture is that most of the time the code is not actually novel. It’s like asking a plumber to describe the physical properties of the pipe they used on a job. They’d say “schedule 40” or “copper” and a dimension. They would not describe the manufacturing process or chemical composition of the pipe. The materials are pretty standard and only require special descriptions for when and why they deviate from those standards.

There are a lot of books describing algorithms. Most network protocols are described in RFCs as well as advices on their implementation. So you are looking in the wrong place. Source code documentation is usually enough to be understood by coders who are already familiar with common algorithms, protocols and APIs or know where to find their description. Only things that are very specific to the project can be described in details.