Probably because, like many other ancient document formats (e.g. MS Office), it was a straight dump of memory structures into a file [1]. Obviously a very bad idea in hindsight (especially given the truckload of deserialization vulns resulting from it), but computers from that age were so memory-constrained that anything else wouldn't cut it, and by the time computers got more powerful the old formats were hopelessly entrenched.

[1] https://www.joelonsoftware.com/2008/02/19/why-are-the-micros...