[Lucas00]

As more and more people open their minds to things other than topps/bowman (very common cards) they realize their own collections are very streamlined and in some ways not unique. Of course there is no problem with that. But for people who eventually get those thoughts, the general next step is going for memorabilia that is unique, scarce, rare.
Pre war you will see has adopted this thought process decades ago, getting pre rookie or early hof material of pre war greats is almost impossible and if possible you seriously have to be a millionaire to consider collecting.

As it is now, in particular 50s and 60s pre rookie issues are in my opinion extremely undervalued. And many multiple 100 times rarer than their rookie standard counterparts.

I'll give one quick example of one of the craziest. Stan Musials M114, an issue of one of the top 5 best hitters of all time that is 5 years before his "rookie card" goes for under $200 and often times under $100. That is comical. You can get original photos that are many times one of a kind for a fraction of a topps rookie card that quite literally is a dime a dozen.