They don't own the streaming rights, Disney does. Avengers is owned by Marvel who is owned by Disney. Disney is streaming it on their own OTT platform - Disney+. They are not selling its rights to others and so Netflix does not have the Avengers series.
Why is there no Marvel movies on Netflix?
Netflix used to have quite a few Marvel movies. The movies were added to Netflix as a part of Netflix's deal with Disney. It ended a while back before Disney Plus launched, so over time, the Marvel movies have been removed from Netflix. So, no, you won't be able to watch the Marvel movies on Netflix.