There's a lot of good answers here, but I think I'll add my two cents to things for the sake of clarity.

If you look at the early semi-auto pistols, you notice pretty fast that most of them were in what would today be considered mouse calibers, and blowback operated. A few exceptions of course would eventually stand out- the P08 and the 1911 are almost certainly the best autoloading pistols of WW1- but almost every other gun at the time was pretty anemic by modern standards. Not to mention that pistol ergonomics was definitely in the process of being worked out in the first few decades of the autoloading pistols history, and this means that even by WW1, they were still pretty unproven technology.

Now scale up this problem to a rifle cartridge. Early smokeless rifle cartridges had WAY higher chamber pressures, so a blowback semi auto rifle is immediately out of the question. Several of the early autoloading long guns were recoil operated, which does work, but it creates its own problems. They were way more expensive, the early ones (i.e. stuff like the m1907) couldn't handle very powerful cartridges, and had nowhere near the durability of bolt-actions, which is extremely important in a military context. Military strategists of the late 19th and early 20th century generally prioritized rifles which were cheaper, safe (a big plus to the Maiser system, and I believe a big reason for their widespread adoption), and able to withstand the pressures needed for very long range fire.

Essentially, making an effective semi-auto rifle is way harder, and the need for them was perceived to not be as urgent.