That is because there's nothing spinning in your drive. It's not a mechanical drive, with a platter
SSD means Solid State Drive. No moving parts, it's just another type of storage, made of non-moving chips.
That's the ideal use for your drive.
The default install for Mojave, even going back a version to High Sierra, will automatically convert the drive to APFS.
That's how it works, and the SSD is better for it.
(If you actually did have a spinning hard drive, there are some steps to follow (somewhere) that will keep the format as Mac OS Extended, even with an install of Mojave. But, you have an SSD, there's no advantage to trying to keep the old format (perhaps it is not possible with an SSD, I don't know), but you have an SSD anyway, so all is good. The SSD with Mojave will be APFS. And, there's nothing to fix. Your SSD will be APFS now.