Why pyranosides do not mutarotate?
In principle, the acetalisation reaction is indeed reversible. Typical conditions to reverse it would be to use $\ce{TsOH}$ or $\ce{PPTS}$ and water.

However, both full acetalisation and liberation of the aldehyde require acidic conditions. Hemiacetalisation — the prime mechanism of mutarotation — proceeds without acidic catalysis, i.e. the inherent (low) acidity of water is sufficient. And to observe mutarotation, you typically dissolve the pure sample in a pure solvent (with no acid added).

If you dissolve e.g. methyl &#x3b1;-ᴅ-glucopyranoside in water and add catalytic amounts of acetic acid, you will observe mutarotation but not if you simply dissolve the O,O-acetal in water.