Is it possible to run more VMs than cores available on the computer?
Why is it not possible to allocate more cores to a single virtual machine then number is available on the host computer?
I am using virtualbox.
Yes. Just the same way you can run many processes.
Because that is how the hyper-visor was developed. Some virtualization programs will fully emulate everything including the CPU in software (see binary translation). Others make use of hardware features on your system like VT-x (AMD-V), which is much faster then full emulation, but includes some limitations. Since you are depending on VM features from built into the CPU, you cannot emulate more CPU then actually exists on the system.
As far as I know, no desktop hypervisor is doing full emulation for x86/x64. Occasionally you will see a hypervisor that does full emulation so you can get a PPC on x86, or ARM, on x86.