Can’t I just use a 1000ml Graduated Cylinder?

You can, but you will need to get creative in finding a stopper that will block the pour spout.  Our Hydrometer Jars are much less in price than a glass graduated cylinder. It is clearly marked with an etched line at 1000ml, it has no pour spout so the stopper fits fine, and there is plenty of room for the hydrometer to fit in the top.