That's because they make no revenue off it. Let's say they're doing all the work scanning, cleaning, and translating the scans, this process takes hours and hours.
When they post it on their website, they might have ads running in the sidebars, or when you click "download" on their site, they take you to a webpage that has ads, so it's basically supporting their website by funding them and their teammembers.
This is just one of the reasons I can think of.
Maybe they also don't like the fact that websites such as mangafox, mangago, mangarock etc. is making large amounts of money through their scans and hard work. (aka they're all sites that run ads on their website too)
I mean I definitely still use mangago for everything so I'm not complaining, but I can definitely see why scanlation sites are possessive over their work.
Even though this is super old I'm going to add my two cents for the people who find this manga from now on. Adding to your point (which I totally agree with), there has been a huge increase in authors finding their work on websites like these. I highly doubt they just see the title of their work and then immediately flame the site. I'm pretty sure they'd probably check a few chapters to see that all their work is uploaded here. Since the scanlator's credit pages are there they might contact the scanlators and ask for their translations to be taken down. Maybe you haven't seen the turmoil within this site in the past few months (e.g. t*ktok) and people complaining that sharing these sites brings more attention to it and therefore a greater chance it will be taken down. And scanlators don't have this issue? That's why many of them have their own websites or blogs instead of directly uploading their work to ripping sites like mngago. It's harder to be tracked on the internet and would you rather have to put 10 minutes of extra searching or no translation at all?
For OP in particular,
Scanlators have every right to claim their translation because they're the ones that took the time to translate it, clean it, proofread etc. Yeah they aren't the author themselves, and should have asked for permission what author would agree to having their works up for free but just in another language? If you're going to enjoy free stuff without having to go through very much effort you don't have the right to complain about anything. If you don't like it then do it yourself. Pay for the manga and translate it yourself.
I do however have a problem with scanlators that use pages/art they found from the internet for credit pages without crediting the original manga/manhua/manhwa/artist that the credit page art comes from. If it's the same art as the chapter that the credit page is part of that's fine because they usually have the title and author of the chapter anyways but otherwise those other authors are equally entitled to the credit. Scanlators, if you want your do not repost/keep the credit page when reposting rules to be respected you have to do it too -_-
Ok, about the "warning" at the beginning of the first chapter; WHY are scanlators being so possessive of their translations lately? This isn't the only manga I see this stuff on. I get you guys want credit and for the most part that credit is always provided, so I don't get why they are getting upset about people uploading "their translations" to other sites people would prefer to read on. (for me I just like to keep all the manga I read in one place and I am used to how this site works.) But they act as if they were the manga artist and are having their work stolen, when in reality they aren't much better in terms of 'theft'. They do realize they don't own the manga they are translating, right? I'm not saying they shouldn't get credit for their work, I'm just saying they need to be less possessive of "their translations" unless they legitimately got legal rights to translate the manga, which would be an entirely different situation.