These are very interesting questions. It deserves an explanation in detail, but I will try to summarize the answer here.
It is necessary to understand that the main role of a witness a technical role. Our main function is to keep the blockchain running and ensure its viability thanks to the decentralization of our different nodes.
Second, we are developers in most of cases and we must meet the needs of the community by improving and creating tools that make the user experience more comfortable and practical.
These functions take up a lot of time and have a high economic cost due to the necessary infrastructure. Many witnesses must use the Steem earned for their work to pay for their system expenses. For this reason, there may be curator witnesses, but it is not normal. In most cases, witnesses who can afford to accumulate SP act as "passive" curators, that is, following a curation trail.
Fortunately, Steem has a very varied ecosystem and there are other roles more focused on curanting and supporting beginners, such as this "World of Xpilar" community or other projects such as @steemingcurators

Regarding the deactivated witnesses. Steem is a living and changing ecosystem. There are witnesses who stop working because they don't have the time or resources to keep the blockchain. Others deactivate their nodes because they start projects elsewhere. All witnesses, present and past, continue to appear on the list and can continue to receive votes, even if they no longer perform functions. Users should be responsible for checking whether the witnesses they support continue to do their job to decide whether to hold or withdraw their vote.