This is a very pertinent observation. I guess one of the reasons is that articles are usually paid by the word, so a writer's contract will cover a certain number of words or the writer will feel justified in asking for a pay rise. Same with photos; with proper publications they are usually acknowledged and paid for, in print or online format. I imagine a video will be subject to the same rules, even if it's simply re-posted from Youtube. So it's not just the limit given by the number of pages, it's also a budget limit.
While an online magazine doesn't have paper and distribution costs, it does have IT/storage/hosting/designer costs. And if any aspect of it is made to be properly interactive, there will have to be one or more social media experts, or similar roles, who are paid to lead and keep alive the conversation with the audience.
I assume that, at least at the beginning of the existence of a proper online-only magazine, the total costs will be comparable to a printed version. Once it's successful and has been going for a while it may feel a bit richer, or maybe just less poor. We shall see.