Of course if Starfleet can build ship that can fly to the other side of the galaxy in four years, why hasn't the Federation explored deeper into the galaxy yet?
The whole speed and distance thing is always so wonky in Star Trek.
I use the following rationalization for myself. It may or may not make sense to you.
Suppose you and 10 friends get the assignment to 'explore' every settlement on this planet with 10 people or more. That is, go to each of them, spend a few hours or days in each, talk with the local chiefs about their current situation, and make a report about them and the most conspicuous features of their group culture and/or the nature in that area. In order to do so, you get a pass that allows you access to any form of transportation, including airplanes.
Suppose I come back in 10 years and ask you how far you have progressed with your task. Odds are that you haven't even finished with you own country (assuming you live in a large one), and dertainly not your continent. Does it matter much, then, that you
could fly to the other side of the planet in just 24 hours, if you want to go about it in a systematic fashion?
The galaxy has about 400,000,000,000 stellar systems to explore. Even if they had a hundred thousand exploration ships (and I'm fairly certain they don't), that would still be 4 million of systems for each of those. So it's going to take
centuries. And technology that could get you to the other side of the galaxy in mere minutes won't be of much help here.
Of course, none of this squares with the notion that it
will cost Voyager 75 years to get back to the AQ, even if they go in a straight line and avoid any detours ...