Slightly OT: how fragile are 3D printed things? Do they break easily? Do they feel fragile? The rods in thi design, might a person break them by accident? How about intentionally?
To answer your main question, it depends. There are many materials you can print with, they're strength can differ greatly. Some are very rigid, but somewhat brittle, others are less rigid, but also less brittle.
Then it also depends on the design, the video[0] below is a great video on designing for 3d printing and the considerations needed for strength.
Finally, print settings matter. Temperature can affect layer adhesion. Infill and wall count can affect strength and so can print direction (see video).
3D prints can be pretty strong though. I'm using 3d printed mounts to hang snowboards on my wall, for example. I've printed plenty of other stuff I really couldn't print by hand. And I'm only using FDM printing, with SLA and SLS printing even stronger parts can be printed.
PLA is not strong and doesn't like to warm up in the sun, Silk PLA (made to just look nice on the shelf) can even be brittle. But there are better materials.
PETG is entirely reasonable, I store my pots and pans on PETG hooks and they're not oversized. The superior 3d printers can print ABS which is above any and all doubt.
There are also specialized materials, such as carbon infused PETG, and there are engineering materials at 500€/kg with some interesting properties. But the gist is that PLA makes you go "this sucks, must be printed", and PETG makes you go "oh this is fine".
Then there are resin printers that selectively UV cure a liquid to polymerize it. Last I tried that sticky goo it was awfully brittle and borderline unusuable. I heard it improved, but that's why resin printing is not everyday parts but figurines.
Under most cases, you won't get the same inter-layer adhesion with ABS, so while you get better impact resistance, under most circumstances PETG will yield more durable parts that won't delaminate under the same stress conditions. For outside use under the sun, you should use it's cousin ASA.
To respond to the OP.. 3D printed parts can be incredibly durable when printed correctly. The parts need to be designed for 3d-printing in mind, like most other manufacturing methods. A 1cm-thick 15% filled PLA slab that has been printed vertically might be easy to snap in half with two hands, but it becomes almost impossible to break with bare hands when printed horizontal, and requires a saw to be cut when filled to 50%+. And this using consumer-grade printers.
I'm using 3d-printed parts for work and at home, some in use for almost 7 years at this point, and the only telltale sign is the layered look.
The rods in the design are not 3d-printed, which makes sense (most plastics would be too flexible, and 3d-printing a rod is always more expensive).
I print on a Mk3s in a photo tent and have had no trouble eliminating layer failures as an issue.
Of course it all depends on you application.
"Strong" is ambiguous. PLA is stiffer than many other thermoplastics (resists bending loads well), but it's not as tough (impact strength is poor/it's brittle).
PLA becomes even stiffer and resists higher temperatures than many other thermoplastics if you anneal it.
You want stiffness in a telescope because deflecting under load moves your optics out of alignment. A telescope has moving masses as well, so stiffness along all axes under load is important.
It's probably more to do with it being messy, smelly, and overly finicky business[0] compared with FDM (but there's zero chance of burning your house down, I suppose.)
[0] levelling the build plate, cleaning the FEP on failures, replacing the FEP, washing the results with IPA (mostly), having to keep the resin warm to get good results, draining the resin when not used for a while, faffing about with orientation and supports is a black art, etc.
It's on the 3rd version, the first one broke on small collision (the bike fell over while standing), the 2nd on a big collision (my wife hit a bus stop :/ ). The 3rd version survived several years of daily use including hauling stuff from the mall.
The key is to split the design into parts that need to be strong in 2 axis and can be weak in the 3rd axis (to work around the layer adhesion problems). So you print 4 walls laying flat not standing vertically. That plus including rope or zip ties for elastic connections where needed - and you can make stuff that is plenty strong enough for most daily uses.
I printed with PLA and PETG and both are ok - PETG is less stiff but handles direct sun better.
Most things aren't particularly fragile - print out a 1" cube in any material at default settings, and you'll be able to stand on it. You probably won't find a way to break it with your bare hands, short of pitching it into a concrete wall.