Exactly, there is no need for nefarious intentions, when time constraint et mild incompetence suffice.
The OOXML format is likely a not very deeply thought out XML serialization of the in memory structure or of the old binary format, done under time pressure (there was legal pressure on Microsoft at the time).
The OOXML format is likely a not very deeply thought out XML serialization of the in memory structure or of the old binary format, done under time pressure (there was legal pressure on Microsoft at the time).