Yes, you absolutely have a point that the Presidency and state governorships have term limits. But there's a reason for that: executive authority is more powerful than legislative authority.
For example, Congress writes broad laws. The President can then narrowly define those laws during his administration through the use of executive orders. In some cases, the President can even use executive orders to declare the President's position that portions of laws are unconstitutional, and therefore will be ignored by his administration. So depending on how much Congress flexes it's muscles, the Presidency gives one person an incredible amount of power.
This is why there are term limits for the Presidency - while the position holds a great amount of power, a person is limited by how much time they can serve in that position.
Legislative authority is different, however. Legislative authority gives a person the power to write laws. However, unlike the Presidency who holds ultimate absolute authority, legislative authority is diluted among 435 Representatives and 100 Senators. All of them take part in the writing of laws, and then bills are passed if only a majority in each chamber votes for it.
So a single Representative and a single Senator does not wield nearly as much power as a single President does. Therefore the argument that they wield just as much power isn't quite true.
So that's the difference between why executive positions have term limits but legislative positions usually don't.