Not completely certain, but likely a rights issue.

If that's the case it's damn stupid.GOG have the right for the game but not for the manual ?

Apparently, the manual and strategy guide were written by some different company, the Stratos Group. Which I've never heard of. And I sincerely doubt they're still in business. It's likely the manual is in some kind of legal limbo.

However, it's quite easy to find the manual by googling it. Replacement docs has it.

Thank you :) +1

The manual is not that useful because it lacks the tables. There was an addendum written, you can probably find a DOC of PDF of it floating around the internet. Or use my open transcript:
https://github.com/HiPhish/Game-Source-Documentation/blob/master/Age%20of%20Wonders/Addendum.rst

It's written in reStructuredText (reST), so it can be converted to a number of different formats. reST is essentially plain text (click on the Raw button to see the source) that follows some basic rules that allow for things like heading, lists, tables and so on. A converter can read the source and generate something more complicated, like HTML, from it.

there is a manual! It's in the age of wonders folder, which is in the GOG folder in the C:/ drive, called quickstart.pdf You can easily access it if you are running GOG Galaxy which has more options than using the web interface.