Plus if you look under the covers of MySQL each "table" is just a file on the disk. Unsophisticated database users feel comfortable with that too. "Sharding", another thing they like, is another attempt to evade using advanced features (not that partitioning is actually "advanced" these days, nor is a semi-decent query optimizer) that other databases take for granted.
Not only was I totally clueless that MySQL innards are represented on a one-file-is-one-table basis, because I'm an unsophisticated database user precisely because I don't want to know how my database works on the inside, I'm just sophisticated enough to know that any attempt to exploit the knowledge that the users table corresponds to a single file will result in my dog being assassinated by data corruption SQL ninjas.
There is a limit to vertical scaling and it becomes more and more expensive.
It boils down to the old question of right-tool-for-the-job and a RAC cluster is not the right tool for most webapp scenarios.