Now why seedless watermelons suck is an interesting story.
The first intentional creation of a seedless watermelon was done in 1939 in Japan. University breeding programs in the southern U.S. after WWII began using the technology to create the first hybrids. In the early '50’s and '60’s multiple tetraploids were created by the university programs and released to private companies to produce seed with.
These tetraploid lines were “greys”. “Greys” were selected to have a thick hard rind for long distance shipping. They were barely red on the inside and tasted slightly bitter. They all sucked for flavor.
A seedless watermelon hybrid is made by crossing a tetraploid female by a diploid male. The resulting hybrid (triploid) has 3 copies of every chromosome and is sterile.
Fertile stable tetraploids take a long time to create - around 15 generations of you are lucky. Seedless watermelons also took a long time to gain popularity in the market. So nobody put significant money or time into creating more tetraploid inbreds for over 40 years.
From the 1980’s when seedless watermelons were introduced until around 2010, everyone used those shitty old tetraploids as 2/3rds of the hybrid. Since 2010 companies have created new tetraploid to use, but a significant portion still use 70 year old shitty ones.