You exactly stated the primary reason
Primary reason is probably water saving.
The labels and government regulations don't check for rinsing results.
So, if there is more water in the laundry, you need less to saturate the load and get better ratings.
Second of all, most of the extraction is done below the about 800rpm border.
For an EU FL, 800rpm equals 70% residual moisture, 1000rpm brings you to 60% and 1400rpm to 50%. Diminishing returns, basicly.
If the laundry is still warm, these numbers improove by about 5% as well, so the spin before the first rinse spins better at equal speed.
And the higher you spin the better you have to balance, the more you wear out the machine, etc.
But keep in mind those slower speeds of todays machines are still often equal in extraction power to washers of the past (500rpm might be half speed today but might have been all the washer could reach a few decades ago).
Last but not least, laundry care.
Spinning laundry with soapy water at extremly high speeds can be damaging to even more robust cotton laundry.
The high pressure of spinning at full speed could lead to white streaks, dulling of color and can make rinsing harder.
First of, the verry high pressure paired with the solving properties of the detergent water mixture can extend the color wear by "forcing out" more color. That's why for example Electrolux does a deep rinse after the wash before the rinse on their "Jeans" cycle.
Jeans for example a verry prone to that, especially cheap heavily dyed ones. They suddenly have white crease like streaks across them after a wash.
On some fibres, especially flat cotton like T-Shirts, a too high spin speed after the wash can force detergent to deep into the fibre.
Those fibres swell while washing, but not to the core. Keep in mind that wash tumbling is far less forcefull then spinning g-force wise.
A verry high spin will not only force the water out of the relaxed outer areas of the fibre, but through the core as well.
Getting that out entirely again requires far more intense rinsing.
That can dull colors by depositing residues in a fibre that is totaly busted oped down to the core.
Finally, a too high spin of warm laundry can set in deformations and wrinkles. Those can act as pockets for detergent laden water.
The cold shock of the rinse water then sets these in, making removal of the residues harder.
Of course, for synthetics, the greatest danger is deformation while to hot. This Whirlpool video puts the synthetic situation well:
That is why EU FLs often use stepped spin speeds and times for interim spins.
Cycles often greatly varry the interim spinning procedures as well. A heavy duty cycle today might to more intensive interim spinning. A delicate synthetic cycle for items like sportswear might not interim spin at all.