I just think that it would be a great idea, since many persons choose Mac as computer.

Please read the game page (answer to your question is in bold):

Raft will release on Steam Early Access in 2018 with multiplayer, reefs and much more new content and features! To be notified when it launches, please visit our Steam page and press 'Add to your wishlist'! We are not releasing any new updates on Itch.io as we are working towards a Steam Early Access release. To read more about this announcement, visit our Blog!

And here is the answer from the FAQ if you're wondering if the Steam version of Raft will support Mac/Linux:

“Will Raft release on Mac and/or Linux?”
We are first and foremost focusing on Windows. We would like to support more platforms in the future but due to certain parts of the game making porting more difficult we cannot at this time give any promises.