Firstly, let's assess if you're in a 'neutral resting position'. Meaning, if you were to glance in the mirror while sitting/standing in what you feel is 'straight', would your body actually be straight? I have a habit of naturally assessing people's postures, mechanics, and symmetry and I can tell you that most people are rarely 'centered straight' -- a shoulder is lower, the neck shifts to one side, a hip is higher, the ribs are rotated, etc.
Now, imagine maintaining these small side shifts with small rotations at the spine (and surrounding joints) over years while you settle into those shifts. Don't you think you'd feel tight/stiff as your pulleys and levers (tendons and muscles connecting to each joint) become inefficient because one side becomes 'overactive' while the other becomes 'inhibited'?
Left pic: sitting with legs out front in resting posture
Right pic: same position after mobilizing the thoracolumbar junction
Above: bridge motion in what 'feels straight' to the patient
If you were 'just tight', stretching that region for 2 minutes on a daily basis should show noticeable gains in a matter of weeks. However, most stories I hear are that people are stretching regularly and remain 'stuck'. Or, I hear they must spend a long time 'getting the body moving' or 'warming up' to feel flexible enough to simply perform basic functional movements.
If you haven't made gains or progress with stretching or resolving stiffness despite regular stretching, your joint is a factor that needs to be addressed.
Best physical therapy practice is to treat ALL aspects of the body while also avoiding "doing the same treatment" each time. There should be regular reassessment while adjusting the treatment strategy as you go!