See the question, Why are the Great Lakes not considered a sea?
The actual answer here is "because the naming of bodies of water is a historical, liguistic and cultural matter, not a scientific one".
And if you know this already, then it's certainly arguable that the question is off-topic (it's about language, not earth science) or that it's primarily opinion-based (why do you think somebody called these things lakes?).
But if you don't already understand this, and have assumed - not unreasonably - that there is some agreed taxonomy of what a lake, sea, ocean, etc., is, then it appears to be a perfectly answerable and on-topic question. Indeed, some people have written answers with helpful explanations of the differences between seas and lakes which, while not universal, do probably go some way towards helping the asker.
So please let's not all rush to close it because, with our superior knowledge, we know that the ultimate answer is not an earth science one - instead, let's use it to explain the misconception, and give partial answers that are entirely earth-science related. Otherwise we're not being helpful, we're removing answers that might benefit others, and we'll probably get asked the same thing again in a year or two.