Petite and tall are treated as adjustments to length. The pattern work involved is more extensive than that, and knowing what moves explains why substitutes rarely work.
Vertical proportion is not one measurement
Height is distributed across several segments: shoulder to bust, bust to waist, waist to hip, and hip to hem, and they do not scale together.
A shorter body is not uniformly compressed, and the torso segments typically change by different amounts than the leg length does.
Grading a petite line therefore requires reducing each segment by its own amount, which produces a genuinely different pattern rather than a shortened one.
Key points have to relocate
Armholes, shoulder seams, waistlines and dart apexes are positioned relative to body landmarks, and those landmarks move when proportions change.
Simply cutting length from the hem leaves the armhole too deep and the waist seam too low, which is exactly what happens when a standard garment is hemmed instead.
The result reads as a fit problem without an obvious cause, because the alteration corrected the visible length and left every internal reference point wrong.
Tall grading faces the opposite constraints
Tall lines lengthen the torso and limbs, but a longer bodice requires more fabric between armhole and waist without widening the garment.
Sleeves must gain length at the correct point, since adding it all at the cuff moves the elbow ease away from where the elbow actually is.
Rise on trousers is the most common failure, because a standard rise on a longer torso pulls the waistband down and changes how the whole garment hangs.
Producing extra ranges multiplies cost
Each additional range requires its own pattern set, its own grading, its own samples and its own inventory across every size within it.
Demand for any single size in a specialized range is thinner than for the equivalent standard size, so the risk of unsold stock is higher.
Retailers respond by offering petite and tall in a narrower selection of styles, usually basics, which is why the ranges feel less complete than standard offerings.
Alterations can only do part of the job
A tailor can shorten hems and sleeves, take in side seams and raise a waistband, all of which address the garment's outer dimensions.
Moving an armhole, reshaping a shoulder or repositioning a dart requires substantially disassembling the garment, and the labor rarely justifies the cost.
Buying from a range graded for the proportion is therefore not a convenience but a structural difference, and it explains the persistent gap in satisfaction between the two approaches.