AFAIK multicore technology is currently used in some automotive solutions, however usually not as
SMP (symmetrical multicore processing) but either
AMP (asymmetrical multicore processing), where each core runs its own dedicated tasks, or as lock-step dual cores running the same code for extra reliability.
This article by Freescale engineers offers some reasons for it being introduced in cars:
Automotive SoCs have traditionally been single core, since not much
computational work or high end applications were targeted on them.
Automotives were simpler, so were the applications and so were the
SoCs. As more and more electronics made room in the automotives, the
complexity of the SoCs kept on increasing. Now the focus is to have
most of the automotive under electronic control.
High end automotives produced these days provide features like
electronic stability control (ESC), traction control system (TCS),
advanced driver assistance systems (ADAS) etc. These features require
complex SoCs at heart which can collect, process and transfer data at
a fast rate from multiple peripherals.
No matter at how much high frequency the single core is operating on,
it will always have performance bottlenecks & challenges while
performing multiple tasks. Single core running on higher frequency
consumes more power. This makes the single core architecture unfit for
ultra low power applications. Dual core based SOC architecture provide
better tradeoff in performance and power consumption than single core
Freescale also has a document Embedded Multicore:
An Introduction expanding on advantages and challenges of multicore chips in embedded environments. For example (on interconnect buses):
Ironically, although microprocessors can perform almost instantly the
sorts of complex complications that decades ago institutions spent
millions on and built rooms for, the step into multicore processing
has brought a simple problem to light: Because the total bandwidth
must be divided among the bus masters,more cores means less bandwidth
Also, with increased bus traffic, the risk of collisions increases and
this lowers bandwidth even further. In short, a bus does not scale
well above four cores.
While automotive industry has its own safety & certification requirements, I think it's somewhat easier for manufacturers to experiment with multicore solutions on ground, compared with planes which have to fly properly in all situations and have much more potential for damage, so multicore adoption will likely be much slower.
However, they may be introduced in avionics too at some point:
The Multicore for Avionics (MCFA) working group, which includes
representatives from BAE Systems, BARCO, Boeing, EADS, ELBIT, GE
Aviation, Hamilton Sunstrand, Honeywell, Rockwell Collins, Thales, and
Freescale, was established to help commercial avionics companies
leverage the performance, power, and size advantages of sophisticated
embedded multicore processors, such as those from Freescale
Report from the 2013 WG meeting: http://onboard.thalesgroup.com/successful-multi-core-for-avionics-working-group-meeting-with-authorities/