That's because of different expectations. For a tape drive, you don't care about latency, power efficiency, random access or write amplification, all that matters is getting as much data as possible on a tape for the lowest amount of investment into tapes. So the tapes are advertised with the amount of net data that you can fit onto it, assuming a standard distribution of compressible vs non-compressible data.

For a laptop SSD, all of these matter. You can't do much compression because compression consumes a lot of power and latency upon access, some compression schemes make random access harder (i.e. compression schemes without an index where you have to scan through intermediate checkpoints or, in the worst case, sequentially through the entire media), and it may lead to write amplification as well if you need to add a piece of data in the middle of a file (basically the issue with shingle HDDs). As a result, no compression possible, and so SSDs are advertised with the raw capacity (or, in fact, they are underadvertised because SSDs need spare block capacity to account for wear).