Same reason the USMC uses the M27 IAR in addition to the M4 carbine. Having a weapon capable of greater sustained fire than the ordinary select fire service rifle is pretty important. See the Battle of Wanat, rifles overheat quickly in rapid fire mode, automatic rifles can last longer. There is a youtuber who does rifle meltdown vidoes and found the standard AK47 could only make it about 270 rounds on full auto before locking up (barrel drooped and separated from the gas tube), but a heavy barreled version that was similar to the RPK lasted something like twice that.
The RPD is a belt fed and light and proved itself in combat not least of which in Vietnam; it suffered from a number of downsides, including having a fixed barrel, but it was pretty damn useful and only gave way to the RPK due to it being a lot cheaper to make due to commonality with the AKM.
Yes a 1920s-30s army was limited by mindset and funding issues (BTW have you ever read the book "Social History of the Machine Gun"? It really covers the mindset issue very well), but it isn't totally outside the realm, as even in 1918 the Germans were writing operations research reports demanding a rifle with only an 800m capability due to the reality of the ranges that combat happened at, while the US also developed the Pedersen Device during WW1 to adapt existing rifles into weapons that were only useful out to 300 yards, but were semi-auto and have a larger capacity magazine as it was judged that the realities of combat made that more useful than the existing bolt action rifle (with the option to convert back if needed for longer range engagements). Even the early work on the US SLR program in the 1920s-30s was about developing what was for the time an intermediate cartridge (yes in part due to the thought that it would be easier to make an SLR with a lighter cartridge).
Germany's efforts were constrained by funding in the 1920s-30s and when it became available they were playing catch up in development of an SLR while having to prioritize rapid rearmament, which ultimately killed the prospects for adopting a new cartridge/rifle that wasn't stamped metal and didn't use scarce strategic metals. So they went the cheap route and developed a Universal MG (not a GPMG), as that was deemed the best bang for their buck at the time of limited budgets that used their existing cartridge. If you want the history of it the Collector's Grade publication "Sturmgewehr!" covers this very well from before WW1 through WW2. They also probably cover it to some degree in their book on the MG34/42 as well, though I haven't read that one.