That perplexed me for a bit too. You can upload to another site...bitchute, YT, Vimeo, etc. and include the link where you are writing your "blog". That will make it so you can see the video on the platform that you uploaded to through Steemit. You can also create a Dtube account which is Steemit's version of YT. I have had issues with Dtube so far and haven't been able to upload content. Plenty of people are doing it so it's clearly user error, I think. Hope that helps.