Back then baseball cards weren‘t mass produced so they were more valuable. Usually when people used to get baseball cards they usually played with them or bought them to get the gum that came with them and then throw the actual cards away. The most expensive card ever sold was a 1909 T206 Honus Wagner card which sold for 3.12 million most likely that will be the most a baseball card will ever sell for. Usually the cards were the most valuable in the 20s, 30s, 40s, 50s, 60s, and sometimes even the 70s. If you find any cards from these years in mint or near mint condition, they might be valuable!
Mass producing is when you produce something in the 100 thousands or even million. that’s why most cards nowadays aren't worth a lot of money or are as valuable. Sometimes if you get a rookie card and that player becomes a star, then it might be valuable but if it’s mass produced, it won’t be worth as much.