Macbook models are mostly "old." One of my co-workers complains about this a lot, but the default narrative ("Steve Jobs left so Apple can't manage to release new Mac hardware anymore") seems silly to me. Obviously Apple can make an updated Macbook or Mac Pro every year or two if they wanted. Maybe it would get worse every time without Steve Jobs, and maybe not, but it's not like they suddenly became incapable of making new laptop models.
There's no way I can know why they do it this way, but: on the whole, I'm better off because of it. I don't have to upgrade my Mac so often. Software for the latest Mac works fine on my older Mac - because they're the same. Less platform fragmentation. Fewer screen sizes to support. Fewer memory sizes. Easy to make sure the latest OS version runs on everything. Fewer wifi variants to have interop bugs. (As a wifi AP vendor, I could actually just buy every model of Mac from the last 5 years and include it in my test bed. We don't, but we could :))
Not changing stuff is amazingly powerful as a product strategy and can lead to huge customer satisfaction.