Because a verification environment built with OOP language like system verilog can be very complex, similar to software. Some hard to diagnose env bugs are because the hardware engineer who developed the env does not fully understands some of the subtleties of OOP language, tools, etc. How compiler handles recursive function calls would fall into that category. Another one is how the language/tool handles memory leak.
Basically, a verification engineer today is much more closer to a software engineer and must understand more and more software concepts.
- Hung