Suggested

Too predictable = too boring

Random safety cars will be welcomed by the managers who benefit and cause a lot of ill feeling from those who potentially have a win taken away because of a random event over which they have no control. Worse still, imagine it's the last race of the season and it takes a championship away from one manager and hands it to another.
I've witnessed managers going ballistic about slow backmarkers with no setup and auto-KERS on unattended but otherwise competitive cars. I imagine there might be one or two coronaries if safety cars were to be introduced LOL.

@Michael
I understand where you're coming from but as I said it would cause a lot of bad feeling with some managers. If it is possible to implement this feature (only the Devs would know how difficult it would be to code) perhaps the best solution would be to allow league hosts an option to either enable or disable safety cars.
This would give all managers a choice, if they want safety cars they find a league where it is enabled, if they don't like safety cars they join a league where it is disabled.

Gustavo was refering to the game and not Formula 1 or GP2. In the game the accidents either would have to be just completely random or based on stats we have in the game. The most likely stats would be reliability and cooling on the car side and maybe composure, experience and/or other skills on the driver side. In Rookie tiers for a lot of teams neither the cars nor the drivers got anything to show in any of these attributes, hence Gustavos prediction especially on tracks that might be more accident prone. 

Still wouldn't work I recon. The starter teams would still encounter a crashfest until they figure out the reason for it and manage to train their drivers, or if it's experience you can't train as such at all, and get their car up to the task with that one design point per race they get. But once their teambuilding picks up speed their cars would suddenly turn into almost invincible.

To get things halfway evenly the system would probably have to incorporate manager level, tier, maybe position of the race in the calendar (because at the first race everyone has to deal with the starting car without much choice while even now at the last race(s) almost everybody in Elite has cooling and reliability covered because they simply can't spend those points anywhere else anyway). Then of course driver and car stats and probably driver fitness and wear of the car and engine.

The programming would be completely manageable I suppose, but I'm not so sure about communicating all this to the users. But maybe I'm just seeing things too negative and I'd be racing in a league without it anyway. Even if it's a weighted chance, a race decided by it just feels like a wasted race, a victory or place gained a mere gift by the game and a DNF just frustrating. Neck on neck over the finish line, that are the races I hope for when attending.