I don't understand the trend of listing mid-high range motherboard with a low range processor. Even the most basic motherboard chipset supporting Ryzen 5000 series can run processor up to 5800x without any issue. It's simply a waste of money to have good motherboard but not good processor to take advantage of it.
Having good motherboard was kind of a requirement back in the days as good VRMs were needed to overclock the processors and/or sustain full load for longer duration of period but things have come a long way since then. These days even the most basic motherboards come with something like 6+1+1 VRM setup which has smoother load distribution and most cabinets are not air starved metal boxes anymore so there is a constant airflow going through the VRMs keeping them cool.
The only use case for a low end processor + high end motherboard that I can think of is in case you will be upgrading the processor soon or if you get the combo as a good second-hand deal.
allow me to clear your misunderstanding. as stated above "for reference", no one is advising the OP to get exactly what is listed. everyone else who contributed in this thread also knows that the chance that the OP will get the specifically advised parts is a variable.
focusing on the specifics you've quoted, i advised on getting an r5 5500 as it costs less but performs basically the same across the board which will save money for other parts. again, as stated "for reference", i went to mdcomputers site and put together a list of parts which would be close to the stated budget on a whim so the OP could get an idea on building a PC around the r5 5500.
as for "the trend", i don't know either. as far as the board is concerned, am4 is known to be a platform which supported upgradability as amd is releasing new cpus for it, even now a new range of am4 cpus are bound for release. a mid range kinda fits in the picture as i was guessing the OP would want to upgrade the cpu in the future since the upgrade path for a 12th gen intel is kinda limited compared to maybe a 5700x3d or a 5800x3d in regards to gaming which the OP has mentioned in his post. yes, you're right that even entry level boards can work with most supported cpus. let's be real, this is suppose to be the OP's first build and wants new parts, you have seen the aesthetics of a mid range and entry level boards, right? with the more populated i/o ports along with looks and appeal work wonders in buying parts for most people, right? i'm not hoping the OP is the sweatiest kind of pc builder who opts for saving money alone, so , making an educated guess would be an affordable mid ranger within the budget. which could even be cut down to a microatx stated above to save even more.
if i'm wrong about that, the OP could get one of those ECS brand A320 board to save money.

(and no OP, i'm not making a point to make you buy the r5 5500, i'm just stating the obvious on why such parts were recommended, you can buy which ever you want)