Star Destroyers do have shielding, the enormous spherical objects you see on the top of them are described as "Deflector Shield Domes".
We see evidence of Star Destroyers actively using this shielding in both Return of the Jedi and Empire Strikes Back.
INTERIOR - VADER'S STAR DESTROYER [The Executor] - BRIDGE
Admiral Piett and a commander stand at the window, looking out to the
battle. They look concerned.
CONTROLLER : Sir, we've lost our bridge deflector shield.
Empire Strikes Back :
INTERIOR STAR DESTROYER [The Avenger] - BRIDGE
The tiny Falcon heads directly for the Avenger's bridge. The
Imperials stationed there are stunned to see the small spaceship
racing low across the hull, headed directly at the huge windows of
the bridge area. Alarms go off everywhere. The Destroyer's
commander, Captain Needa, can scarcely believe his eyes.
NEEDA : They're moving to attack position! Shields up!
As far as TIE-Fighters are concerned, the Empire's intention seems to be to build them as cheaply as possible. Adding additional systems (such as deflector shields, hyperdrive, life-support and ejector-seats) is technically feasible but would add to the overall cost per unit.
For the record, Vader's own 'TIE/x1' did have a full deflector shield and a hyperdrive, at least according to the LucasArts "Star Wars : Rogue Squadron 3" game:
The prototype starfighter used by Darth Vader at the Battle of Yavin
and the basis for the frightening efficient TIE Interceptor, the TIE
Advanced X1 is easily recognizable due to its distinctive bent wings.
Unlike all other TIE Fighters, the prototype is equipped with a
deflector shield generator and a modest hyperdrive system. The TIE
Advanced is armed with fixed twin heavy blaster cannons, but has no
life support systems.
The Star Wars Technical Journal article about the TIE fighter goes into a little more detail behind the thinking. Bluntly it boils down to three main factors, weight, cost and the perceived low value of a pilot.
The most common of these is the TIE fighter, a deadly match for any
starfighter in the Rebel arsenal. TIE fighters are cheap to produce
(hence their great number) and operate quite efficiently, with no
defensive shielding or ejection systems for their pilots (which would
add to the ship’s overall body mass).
This reflects the unfortunate Imperial viewpoint, with no regard for
the value of life, that like their fighters the combat pilots of the
Empire are plentiful and easily replaced.