With today's technology and the capabilities of S-rank healers, it's very easy to heal a person to full health without leaving even the smallest scars on their body.
Your skin can be repaired to the point that it will seem like new!
If a scar is still present on someone's body despite all these technologies available, that shows one of two things.
Either the person didn't get healing on time, or they did not have the resources to hire a good enough healer or get treated by good hospitals.
Another reason could be that the scar serves as a 'representation' of a past failure that the individual has intentionally chosen to keep.
With Ashtel's level of strength and money, obviously he had enough resources to hire dozens of S-rank healers to help him in a moment of trouble.
He's even friends with an SS-rank healer, the elf queen, which means he never had to worry about healing at all.
