Defending ourselves from aliens we can't rustle up rookies who can aim something like on-target?
You want too much. XCom has the best scientists in the world. In a year's time they came up with an interplanetary vessel armed with fusion weapons. Even if it is claimed that is just stolen tech from the aliens, they also came up with several laser weapons all on their own in about a month. Ronald Reagan spent years and billions of dollars with thousands of scientists on his "star wars program" and didn't even get a viable laser pistol design. XCom also has the best engineering and machine shop operation in the world. These guys built those light weight, accurate, no spare battery packs required laser weapons. But even more incredible, they built that fusion weapon armed interplanetary vessel in about a month all onsite mostly out of recycled junk. In comparision, Boeing which is a much bigger organization with a much bigger budget and much more experience designing and building aircraft couldn't get their software right on their 737 Max nor get their Starliner to the ISS.
As for the soldiers, well see the best I can figure it, XCom doesn't use trained soldiers. After all, if they started with special forces guys or the "best of the best of the best," these rookies wouldn't get over 3 times stronger over the course of a year. Xcom also doesn't seem to have any basic training capability either, instead preferring to do "on the job training" during missions. Unlike the mib guys, xcom never developed a neuralyzer. I think XCom drafts or conscripts into service any civilian that reports any ufos or little grey men in order to keep the whole alien thing secret. If a bunch of draftees get killed fighting aliens, well at least they can't spill the beans about flying saucers. If XCom is successful and the old surviving veterans years later tell stories about fighting big green men, secret government agents can discredit them by reminding the public these guys also claimed orange snake people terrorized their neighborhood in the late 90's. Similar to how in the movie "Independence Day" for years no one took the character Russel Casse, played by Randy Quaid, seriously about being abducted by aliens.