Not sure who these stat people are that have never attempted to address this, but believe me, stat people have attempted to address this. The problem is the small sample size in the postseason. I know that explanation doesn't sit well with you.
Ortiz has played in the postseason in 9 different seasons. Here is the difference in OPS between the postseason and the regular season for each of those years:
2002, -.184
2003, -.316
2004, +.295
2005, +.082
2007, +.138
2008, -.201
2009, -.627
2013, +.247
2016, -.549
So, in 9 postseasons, Ortiz had a better OPS in the postseason than he did in the regular season 4 times. He had a worse postseason OPS 5 times. With that, can you really say that he was clutch in the postseason?