Polarizers aren’t built into lenses by default because they’re useful only in specific situations and they always come with tradeoffs.

A polarizing filter typically reduces light by about 1.5–2 stops, which can be a problem when you need faster shutter speeds or lower ISO. You also don’t always want the effect: for example, many photographers wouldn’t use one for portraits, and on wide-angle shots it can make blue skies look uneven.

Cost is another reason. A good polarizer is not cheap, so permanently including one on every lens would raise the price of every lens whether the buyer wants it or not.

In practice, it makes more sense as an optional accessory that you can add only when needed and rotate for the desired effect.

For buying one, the answers consistently recommend choosing a well-made circular polarizer with good coatings rather than the cheapest option. B+W and Hoya are both commonly recommended brands.