I agree. So the question is, why didn't they do that? Loads of other Martials and even loads of Ranger subclasses have abilities to do a small amount of extra damage like that without it being a spell. If anything, it's the norm.
So why did they make Rangers have to spam a weird concentration-free, always-known (but not always active!) version of a spell? It doesn't fit the theme. It's not tradition. It's just weird.
My feeling is it's essentially "trolling" from WotC to see how people react. If there's no pushback they're likely to keep going with this sort of "let's make non-magical stuff into spells".
I suspect it is just something that has become a thing onto itself. When they started work on 5e, they could have made hunter's mark and hail of thorns and conjure volley and zephyr strike into individual class features or invocations-with-another-name or in some cases just bonuses (like expertise or most fighting styles). They didn't and made them into spells. Now they are in a position where they can keep adding qualifiers to the spells, or they could go back and reinvent the wheel. Given how much they seem not to want to rock the boat (so much as polish the oar handles and trim a few heavy bits high up on the water line), I don't think they really want to risk a complete rebuild.
It's kind of like Sneak Attack -- when they were making 3e and all the classes were (in theory) supposed to be balanced at a given level instead of the thief being weak but levelling quicker, it made as much sense as anything else to take the cool-but-rarely-used backstab and turn it into something you tried to do as much as possible. But there were all sorts of consequences (rogues focusing on getting as many attacks per turn as possible and running from undead and constructs) and it might have made sense to ditch it for something else, but by then it was a thing onto itself and instead the design goal for the next version was to make a better version of Sneak Attack. Same with Rangers and spells in general (their spells could have become non-spells the instant AD&D got a skill system), and these 5e-specific 'spells for no particular reason' today.
The more I think about it, the more I think that 50% of the problems people have with a spellcasting Ranger could be solved if they just put in a ribbon explaining how Ranger spellcasting is different and making it more in line with both people's mental image and the lore.
I think if that had happened earlier, it might have worked. Say in an oD&D expansion or AD&D there had been an aside next to rangers or clerics saying 'these abilities are presented as spells to standardize structure and minimize rules conflicts. Many of them can be reinvisioned as acts of extraordinary skill or capability. If you and your group do not like the concept of a godly-infused priest being the primary healer type in your campaign, they can instead be highly trained medics and the spells as a limited resource as a limited amount of chances to defeat death before their skills are spent [weeble wobble, hand-wavium on the rest of the cleric spells, I guess undead have bad memories of doctors]' I think people would be okay with it and the game would have progressed with a level of spell/non-spell transparency. At this point? I don't know. They tried to do that with spells and psionics in early 3e (and spells and bo9S maneuvers in late 3e) and it was a bridge too far for some (unclear if that was many or a vocal minority).
The other 50% could be solved by taking out the weird damage-dealing spells like Hunter's Mark and Conjure Volley and just making them Ranger features, like they really should be in the first place. This will also make it easier for the features to not just suck horribly like Conjure Volley since they don't have to balance half-caster spells with full casters.
Ranger features or ranger-only spells (for this purpose, I don't think the difference is large). The decision to have universal spells and inter-class sharing is a huge part of all this. If a ranger is okay casting it at L9, the cleric or wizard has to at L5 (unless it's on an unshared list, and then what even is the point of bards and all these feats to borrow from each other?). Even beyond that, you have to balance between someone who is giving up a single mace swing/cantrip to cast the spell compared to the person giving up a multiple-attack attack action (perhaps boosted by feats which sit idle whenever you don't attack). It's a non-trivial lift, so I'm not surprised it doesn't always work, but I am still a little intrigued that that's what they went with for the edition