Why don't modern aircraft have landing gear that can swivel to compensate for crosswinds?
There is one aircraft I know of that can do exactly what you state, the B-52:



This plane has steerable dual-bicycle gear which allows the crew to point the gear along the runway while the fuselage is "crabbed" up to 20* off the runway centerline. AFAIK the gear are slaved to the ILS; you dial in the OBI to a course and the gear will point that way even if the aircraft points differently.

The reason this was done is primarily due to the design of the rest of the aircraft. It's a very "dense" design, cramming a lot of weight into a very small package, requiring massive wings and tail just to keep the thing under control at altitude. These surfaces then become huge sails at lower altitudes, and with the high, anhedraled wing arrangement on a relatively low fueselage, there's not much room for error. B-52 pilots often say you have to fly 2 seconds ahead of the plane, it's that slow to react to pilot input. So, classic "de-crabbing" techniques involving a hard rudder with counter-aileron just aren't going to work with a BUFF. You have to land crabbed and correct after touchdown, and on a dry runway that isn't a good idea either with an aircraft of this size.

Modern airliners, for their size, are much easier to maneuver on the whole than the BUFF. Really big ones, like the 747, actually have steerable main gear as well, but AFAIK the 747's mains can only be actively steered on the ground, and because they counter-steer to the nose gear (helps bring the tail around corners; imagine driving something about 4 times longer than an 18-wheeler) they would actually be pointed further off the runway line when de-crabbing.

Four- and six-wheel main gear systems used on larger airliners (777, 787, A380) actually have a small amount of castering built in; if you watch YouTubes of crosswind landings (there are several) you'll see that the rear two wheels of each gear touch first and actually twist the entire "truck" into line with the aircraft's actual direction of travel. Then as the plane settles on the gear and the fuselage lines back up with the runway, they'll straighten. Planes with one- and two-wheel main gear don't need this as much (and such a system isn't as effective as the first point of contact of a tire is directly level with the gear strut instead of behind it).