For reasons of literal science that are beyond the scope of this Q&A Wiki it's fair to say that technology involving Time is not like other technologies. It's entirely possible that due to it's inherent nature one may never be able to truly control Time Travel. It's again possible that Time Lords only gained the control they have by being physically altered by "The Time Vortex". To speculate: Time can't be controlled by humans using our limited technology. Time Lords can because they are at least partially creatures of Time themselves..
Perfecting Time Travel as a goal has limitations: A) Time travel is an extremely complex and tricky thing to control. Even without the Time Lord's locks in place, time travel is still very hard to perfect.
B) Technology doesn't always move forward, sometimes it stagnates, or even moves backwards.
C) The technology required for "perfecting time travel" could be within reach from a technical standpoint but never explored for ethical ones. In other words the scientists could do it but never will because it would be abused. Given the nature of scientists to explore their curiosities it's also likely that a governmental authority is responsible for initiation a ban on this edge technology