Conferences / other publication venues don't require you have a PhD to submit to them. However, figuring out what a good novel research problem is and solving it rigorously is much much easier in an academic environment --- both because you are financially supported to work on said problems and because you have a community of people to tutor you and give you feedback.
I think the financial part is important. Funding in academia is interesting to say the least. Especially when it comes to grant funding, which can often have interesting requirements that are impossible to have privately or as an individual.
