OTS works with some things like training a/c, gliders, stuff that doesn't need to be flung into a warzone and stay in serice for 30 years.
The requirements of warfare can be too complex and ever-changing to support fully-COTS solutions hence the need for "bespoke". It's a difficult balance.
There again there are countless examples of perfectly suitable COTS systems, aircraft etc. that were/are perfectly suitable until the MoD got their clunking leaden fists involved where they didn't need to.