When FASTA was invented in 1985, generally sequencing reads would be about half that.

The simplicity of FASTA seems like a dream compared to the GenBank flat file format used before then. And around the year 2000, less computationally-inclined scientists were storing sequence in Microsoft Word binary .doc files.

A lot of file formats (including bioinformatics formats!) have come and gone in that time period. I don't think many would design it this way today, but it has a lot of nice features like the ones you point out that led to its longevity.