system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How come solid state drives (SSDs) are mostly only available at storage spaces of 64, 128, 256gb and hard drives are available at more "rounded" spaces like 150, 500, 600gb, etc.?

Sorry if this is terribly worded, but I couldn't think of a good way to word this.
gpt
Let's say you have a swimming pool and you want to fill it with candy.  How many pieces of candy fit?  Maybe it's 1,000,000 pieces.

You decide that you want to fit as many pieces of candy into the pool as possible without making the pool bigger, so you make the candy smaller (to your five-year-old self a piece of candy is a piece of candy, no matter what size it is).  You start finding ways to make the candy smaller and smaller until you can fit 1,500,000 pieces of candy in your swimming pool.  You tell all your friends about that feat, then you go back and try making smaller and smaller pieces of candy.

That's an analogy for the manufacture of hard drives:  there's a certain physical space that they have and they're trying to fit as many bits on a platter as possible.  When they hit certain milestones they push that design into production.  There's no preference for numbers that are nice and round in binary, so they stick to numbers that are nice and round to humans.

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Now imagine you go into the candy production business. You release a pack of candy that has 8 pieces of candy in it.  It's popular, but all the kids on the block want more pieces of candy.  To satisfy that demand you release a package that just has two of the previous packages in it, wrapped together.  This new 16-piece package sells great, so you fix up your machines to make the 16- piece packages the standard.

Over time the demand for candy increases so you do just what you did before:  you tie two packages together and sell that as a new product.  Now you're selling 32-piece packages (which we know are just two 16-piece packages tied together, each of which are two 8-piece packages).

Repeat this cycle several times.  As you move to larger and larger packages you streamline the packaging for the smaller packages ("this unit not marked for individual sale").  Eventually you're selling 64-billion-piece packages and 128-billion-piece packages (where "billion" here is actually 1024\*1024\*1024).

This is the analogy for flash memory (SSD) as well as things like RAM.  You have a design for a single bit of data storage, then you copy/paste that and add a bit of control logic and you have a two-bit memory cell.  Then you copy/paste that and add a bit of control logic and you have a 4-bit memory cell.  Repeat that process a few dozen times and you may arrive at an 8 GB memory chip.  Put a few of those on a board with some control logic and you have a 64 GB drive.

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The real key difference between these two setups is that with hard drives you have a blank slate on which you try to write as much as will fit, while with flash memory you're constructing the memory cells individually using a repeating pattern.