Seeing 'these days' in the question makes me feel old already... this has been the case for quite a while now.
The short answer
Simply because it's been that way for a while now, and the industry has settled their practices to go along with the broadcasters schedules to maximize profit.
The long answer
Note: A lot of my input here is based simply off years of watching anime and the industry, these are my personal observations dotted with editorial input I have also seen through the years.
A good insight into how the volume of production of anime has been handled over the years comes from Justin Sevakis' (now defunct) ANN Q&A corner Answerman back from 2013 and again in 2016 (thanks ahiijny)
Scott asks:
Something I've noticed recently is it seems it's much, much more common to see shows being only one cour or split-cour rather than two or more cours compared to like a decade or so. Why is that?
Back when most anime was actually aired during human waking hours and TV networks actually paid for the honor of broadcasting them, TV anime was beholden to its sponsor. This was often a toy company, or a candy company, or even a record label. The sponsor would commit a large sum of money up front to produce the show, and the anime producer would make it, often integrating that company's products into the show itself.
This way of doing business, where all that money was guaranteed up front, was very slow to react to market forces. Sponsors wanted to get their branding out for as long a period as possible, so naturally they'd want to back a long-running show that would stand the test of time. But if the show bombed, and it often did, the lead time for making anime was so long that work had already started on the second or even third seasons before everyone realized they had a big ol' dud on their hands. The sponsor might want to pull out and stop wasting their money, but with so much of the work already done, they'd still have to pay to finish months worth of anime that nobody would watch. It was a huge waste of money.
Now that late-night anime is the norm, the TV networks treat the shows like infomercials, and production committees making them are mostly interested in selling DVDs and character goods. Since most TV anime are now essentially glorified OAV series, the producers no longer feel the need to plan for a show to be ridiculously long without knowing if it'll be a hit. So this way, they plan shows a season at a time, wait for them to air, and see if they hit. If they hit, THEN they'll do a second and third season. It's just a whole lot less wasteful that way.
This still holds true today. Nowadays, these sponsors' shapes are in the form of fidget spinning evil masterminds production committees and the schedules much more tightly controlled by having networks be part of them. A lot of anime nowadays is produced this way ever since the start of the millenium. Your Arifureta example is made by a committee composed of Overlap (Mixed Media planning and publishing), Hakuhodo DY Music and Pictures (Production and distribution), FuRyu (Games and multimedia content), AT-X (Broadcaster), Sony Music Solutions (Sound), Tora no Ana (Retailer), Bandai Namco Arts (Animation) and Bushiroad (Promotional items, etc.) Things in parenthesis are just an educated quick guess on what each party would bring to the table.
Why production committees? In a word: diversification. Anime in itself isn't a recipe for profit, so bearing the risk of investment between a bunch of companies is better if a title flops. Also having companies that specialize in many different things like music, toy making, casting, etc brings expertise from many areas when coming together to make what they like to call a "Media Project". This model has its pros and cons, but it tends to favor the committee. Sakuga blog has a nice write-up about it.
Now that we know this, back to lengths of cours for series.
What production in cours brings to the table is mostly stability and risk reduction. If Arifureta is truly a flop, economically, the committee will probably cut their losses and not renew the project for more mixed media projects and move on. Trying to dedicate resources for a long standing series with a high risk factor would be silly on a economical standpoint.
But some companies have works that they have faith in their potential or enough monetary backing to make series that are longer than 1 cour. For example, Kodansha had quite some faith in Tensura last year to make it a back-to-back 2-cour series with enough leftover content for another season in 2020 if it did well. From what I've seen, it certainly did.
Overlord, One Punch Man, etc. are in a similar boat, and dividing it into cours gives the committees flexibility to either drop anime that's doing bad or simply continue making more with some time in between to recoup and restart media campaigns, give animators time, plan events, etc.
Of course there are other factors that could affect the length of anime like how some commenters mentioned supply and demand, waiting to catch up to source material or even legal issues, but I think the committee/cour structure is the main reason for this.
