TL;DR: It is because the SSD is lying to you and saying the write is done before it is. It can't get away with the same thing for reads.
The longer version of the answer is write caching.
Lets start with the QD1 case. The SSD will report the write as finished to the OS once it has received the data and saved it in a cache locally on the drive, but before it has actually written it to the NAND. This makes a big difference because actually writing data to NAND is quite slow. For reads it actually has to read the data from NAND before it can send it back (unless it has read it earlier and still has it in cache, but that is very unlikely with random reads).
The downside of this is that in the face of sudden power loss there can be data loss of data written to the SSD but which hasn't made it to the NAND yet. Some enterprise SSDs include a super capacitor which stores enough power to finish writing the data in cache to NAND in case of sudden power loss.
You see the same thing for hard drives because they are also doing write caching. They are just not being nearly as aggressive about it. Why is the SSD so aggressive? To answer that we need to move to consider the QD32 case, which is both more complicated and more interesting.
It is not true what you say that random reads are generally faster than random writes at QD32. It depends a lot on which particular SSDs you look at.
If you look at 4k QD1 random reads on many SATA SSDs they all seem to perform in the 20-30 MB/s range. Why is that? It is because 4k QD1 random reads is mostly about latencies and not throughput. The latency comes from three parts:
- The interface latency of SATA/AHCI which involves telling the drive what to do and sending the data.
- The controller itself has to figure out what to do with the data and instructions it has received.
- The time it takes to actually read or write the data to a NAND die.
Neither 1. or 3. changed much in a long time, and that is why the 1k QD1 random reads didn't change much either.
The recent move in SSDs from SATA/AHCI to PCIe/NVMe has greatly cut down the latency of 1., which is why certain m.2 and PCIe SSDs recently have show great improvements here.
One thing an SSD controller can do to greatly help with the latency is read or write to multiple NAND dies in parallel and that way mask most of the latency of 3. If you are doing QD32 4k random reads with NCQ the SSD can service the read requests out of order and make sure it is reading from as many NAND dies in parallel as possible.
For QD32 4k random writes the SSD does something called write combining. When a lot of small write requests comes in the SSD controller caches them locally and when a big enough buffer of writes has built up the controller splits it into nicely sized chunks and writes the chunks to multiple NAND dies in parallel, again to help mask the NAND latency. Another advantage of write combining is that most SSDs nowadays have a page size (smallest amount that can be read or written) bigger than 4k, and combining writes until you get up to the page size helps avoid lots of write amplification. It is in order to do these thing that SSDs are so aggressive in write caching.