Blame today's commercial-obsessed society.
In the late 1960's/early 1970's, a full hour-long TV show ran 50-51 minutes, from opening to closing credits. You can see this on episodes of GUNSMOKE or STAR TREK from that era.
TV sitcoms that were 30 minutes long would run about 25 minutes, maybe 25 1/2 tops. Maybe some lasted 24 and change, but they were all about 25 minutes give or take.
Fast-foward.....to the late 1970's/early 1980's.
A prime-time show like DALLAS is now running about 49 minutes during an hour-long show. This is during the ORIGINAL broadcast -- any re-runs during the year might cut it down another 90 seconds or so. STAR TREK: THE NEXT GENERATION comes out in 1987 -- syndicated, an hour-long episode only runs about 46 minutes start-to-end. A few years later, DEEP SPACE 9 and VOYAGER come out -- 44-45 minutes. When the last STAR TREK series ENTERPRISE comes out in 2001, it's down to 42-43 minutes.
The 30-minute sitcoms ? Well, the 25+ minutes out of 30 that we got with ALL IN THE FAMILY and SANFORD AND SON were also proportionately cut. SEINFELD, the quintessential 1990's sitcom, runs about 23 1/2 minutes uncut. But TBS or local reruns will chop that down to about 22 and change. Now you know why TV shows today, esp. the 30-minute sitcoms, have opening and closing credits interposed with the show still continuing (ala SEINFELD).
Net-Net: Hour-long episodes today like "24" have about 5% less time than 20-30 years ago and about 15% less than 40+ years ago. For 30-minute shows, the cut is relatively less: just under 10% compared to 40+ years ago and 5% compared to the 1990's.