Why can't ransomware practically use RSA to encrypt all files? I understand that a few ransomware have used an RSA public key to encrypt all files belonging to the victim. This is a bullet-proof system in terms of its security because the private key is always safe with the hackers.

But most ransomware use the hybrid encryption approach and during that they make mistakes allowing security researchers to build decryptors. The attractiveness of this hybrid approach lies in the speed of symmetric encryption. But because the hackers makes implementation mistakes in this approach, why don't all hackers switch to asymmetric-only encryption? It seems easier to implement programmatically than the hybrid approach. Is slow speed of RSA the only factor the limits the hackers? How slow is RSA when compared to AES for, let's say, 500 MB of data on an average i5 processor?

I have searched the Internet looking for a comparison between speeds of RSA versus AES for bulk data encryption in modern computers but found nothing. I understand that criminals cannot be reached for comment on why they don't use asymmetric-only approach for bulk encryption but I am looking to understand what could be the practical reason behind why bulk encryption with RSA is so undesirable.
RSA is really very slow compared with symmetric ciphers. 
You can check this yourself running e.g. openssl benchmark:

openssl speed rsa


On my machine (and with openssl 1.1.1a) I get 

Doing 2048 bits public rsa's for 10s: 330640 2048 bits public RSA's in 10.00s


so we can do ~33k encryptions per second and the size of the encrypted block is less than the modulus, so less than 256 bytes. If the attacker wanted to encrypt 1GB file, it would take about 2 minutes. 
On the other hand, 

openssl speed aes


shows that AES-CBC can encrypt ~300MB per second

type             16 bytes     64 bytes    256 bytes   1024 bytes   8192 bytes  16384 bytes
aes-128 cbc      94234.08k   136019.99k   137980.25k   304759.81k   304166.23k   315654.14k


so the same 1GB file would be encrypted in 3 seconds. 

For the attackers, speed is important, since the faster the files are encrypted, the less likely the process will be interrupted.