MySQL was generally (pre 8) optimized for point queries on primary keys. So rows are stored in the PK index, the PK index is a clustered index. Everything more or less falls out of this.
To be fair this probably covered probably 9x% of production queries, like high-selectivity index-covered filters and sorts (with a limit). Even today most planners will still use an inner loop.
It's been interesting to watch how different planners have evolved over the years. SQL Server 7 in 1998 launched with features that MySQL and Postgres wouldn't catch up to until the late 2010s, but they had other features and quality-of-life improvements that made these edge-case optimizations hardly noticable.