If you buy a Magic pack, you’re buying “eleven commons, three uncommons, and a rare or mythic rare, with a chance of getting a foil card at any rarity in place of a common, and (in some sets) a very small chance of getting a promo card in place of a common.” The guaranteed baseline hasn’t changed from “eleven commons, three uncommons, and a rare,” but you have a higher chance these days of getting something slightly above that baseline. It’s nothing like the random distribution of rarities seen in many digital CCGs.
As far as the “is this gambling?” thing, if you buy a Magic pack, you’re specifically paying for “fifteen Magic cards, with a predetermined baseline and distribution of card rarity, and a slight chance at something slightly above that baseline.” Any cash value attributed to individual cards comes from players and the secondary market, not from WotC saying “this card is worth $10, this other card is worth 50 cents.” Card rarities also have an actual gameplay reason to exist - Limited formats rely on some cards (which aren’t necessarily “money cards” at all) being printed at rare or mythic rare so that they don’t show up every time, for variety and often for power level reasons (many rares and mythic rares aren’t powerful enough for Constructed, but are devastating in a booster draft or sealed deck). Cards at common are also often valued by players because of the Pauper format, which only allows cards that were printed at common at some point in the game’s history. Then you have cubes (custom draft formats; a very popular variety to make is a “peasant cube,” with no cards above uncommon), Commander (where just about everything sees play), and a good number of other ways people find value for the “junk cards,” no dusting required.
Oh, and of course there’s the bit where physical card games don’t have a direct link to your bank account the way digital games do…