While the other member states allow the parents to be included under the family reunification
I don't think any EU member states allow parents of adult EU residents to be reunited except under the most extraordinary of circumstances, generally to help support a minor child.
I should focus on visitor's visa in this case.
Your parents would have to get that visa on their own merits. Since they are destitute (dependent on you for income), they will have a hard time convincing a consul that they have the means to support themselves during their stay. If you sponsor them, then they will not be able to stay longer than the length of the visa. They would not be allowed to seek work or apply for residency, as that would violate the terms of the visa. In any case, it is going to be virtually impossible to prove to the consul that they intend to leave the country when their visas expire, as they clearly have no intention to do so.
The reason you find so little about it on the internet is that what you are trying to do is next to impossible.