Sometimes bottle designs (even custom mass produced ones) don't have options for lids that come with foil seals. Other times it just isn't cost effective, adding even 2 cents to the cost of production on an oil that retails for $4 can be an issue.
There are other risks too. Most people don't remove the entire seal, just puncture and pour - whether it comes off when you make that first puncture, or as you are pouring - a small piece of the seal getting into the oil can be a real problem, especially since you are usually pouring your oil directly into the engine and there is a chance it won't make its way to the filter. Imagine if it blocked an oil gallery or even worse got pulled into a bearing. IMO it's just not worth the risk.
There may be some good BITOGers who can remove the seals without any issue but a rule in marketing is to follow murphy's law. Oil without seals may leak if you put the bottle on it's side (which if you do that you are asking for it IMO)but they keep us safe from the logically challenged.