First of all, because Word is one of the most commonly used softwares, it is convenient for HR to view resumes.
Secondly, it is easy to read a resume in Word. Now HR has to read more than N resumes every day when recruiting. If you use PPT and other software to make a particularly fancy and cumbersome resume, HR simply has no patience to read it.
In addition, if you are not applying for an art design position, your resume should be concise and clear.