Because the hits and hurries lead to bad passes which lead to INTs far more often than sacks lead to fumbles.
I like hits. My bone of contention is chiefly with how PFF values hurries relative to sacks and hits, though I think there the 1.00:0.75 valuation ratio of sacks to hits understates sack value.
As to your statement, are you sure hurries lead to more turnovers than sacks (i.e., the strip sack) on a per incident basis in measuring the relative valuation of the plays? I'm not and in fact I doubt it.
Forced fumbles is a stat that is not given much credit because of the situational oddity. An INT is definitively a turnover to be credited to a single player. A forced fumble may not result in a turnover and the guy forcing it is generally not in position to recover it. I think there's a tendency to think of the recovery, and therefore the turnover, as matter of serendipity, and since the turnover cannot be attributed to a single player.
Using ESPN's stats, the league in aggregate forced 483 fumbles last season with 285 recovered for a 59% recovery rate.
While the recovery may be a matter of serendipity in terms of the recovering player being in the right place at the right time, the guy forcing the fumble has made an impactful play 59% of the time, and that ain't serendipity. On balance, a forced fumble is worth more than half of an INT.
Here's another question. When a defender leaps at the line and tips a ball does PFF credit that as a "hurry" when he didn't actually hurry anything? I don't know, but it strikes me as a whole other category of defensive play (pass rusher passes defended) just as strip sacks should be different category. Some guys have a knack for such plays, others do not.
There's something interesting in the following example of PFF pass rusher ratings. This one presents a ranking of their top 25 pass rushers from "8 months ago". While not dating web pages is a digressionary pet peeve of mine, we can infer from the Chandler Jones notes that this data is through week 7 of the 2016 season:
https://www.profootballfocus.com/pro-the-nfls-top-25-pass-rushers/
The impact of strip sacks is mentioned once under Von Miller's write-up. This is evidently a subjective observation because the value of the strip is not reflected in the numerical rankings because a sack is, well, just a sack in the player scoring world according to PFF.