But, there's nothing that says the explanation can't come from a very real human drive to explore and not from a built in "convenient" technology from within the canon of the previous games. This beginnings of the ME:A story can be set in any time period after today because we as a species have been driven to develop the technology to explore since our very beginnings. To make this a "Mass Effect" story, they don't really need to build on the old canon of the the first ME Trillogy... Humanity isn't limited to that... why limit Bioware to that now? As for making the old canon plausible... the old canon was never really plausible... we all had to make many leaps of faith to get into the Mass Effect Trilogy to start with... what is so unconscionable about making the leap of faith that humanity in all the possible amount of time from today that Bioware might imagine it would take could conceivably discover and create the technology needed to explore our universe on our own? Why does it have to be Reaper Tech handed to us on a silver platter just because that was how the first ME Trilogy went?
Actually, there is. Humanity, and the rest of the galaxy, literally does not have the capacity to build a drive that supports intergalactic travel. This was the case for the entire trilogy. If the ARK actually does depart from the galaxy pre-ME3, it will have to be some unknown technology that justifies its capacity to go beyond known technological limits. The reapers, and specifically Sovereign, are the obvious and most common sense explanation.
Humanity isn't the most technologically advanced species in the galaxy. The asari, turians, and salarians are still ahead of them, which makes it even more ridiculous that humanity would somehow surpass them and play into that erroneous generalization many make about ME1-3 being a depiction of humanity's "superiority." Heck, the reapers made sure that all species developed a specific way by forcing them to use their technology so they could be harvested accordingly.
Maybe because this IS Mass Effect? This in no way "limits" BioWare. Rather, it allows them to legitimize this turn and further build upon the foundation they've already made. The issue with your premise is I'm under the impression you assume the ARK was constructed after ME3. I do not believe this is likely for the simple fact, of which many have already stated, that BioWare wants to avoid the endings. No, it doesn't matter how many years into the future BioWare goes. The endings cannot be resolved due too far too many divergent and galaxy-changing consequences that make it unwieldy for BioWare to make a cohesive future. No, BioWare is not going to canonize an ending either.
Perhaps because the reapers literally made all of the technology that allowed the galaxy to evolve? Reaper technology is and continues to be far superior to any other technology the races of the Milky Way have ever created. Reaper technology has no rival and it wouldn't make sense that humanity, alone, would devise a technology that can rival or is better than reaper tech just to rationalize going to a new galaxy. That would make a mockery of not only the original trilogy, but the IP as a whole. Whatever people's thoughts may be on the reapers, I seriously doubt BioWare is just going to ignore them when they are an obvious solution for how the ARK was built and how it was able to escape the reaper invasion.