There is another article on this site which tries to explain why a tatoo is haram but I don’t think it does a good job. Two points are made: 1.It prevents Wudu/ghusl 2.It changes the creation of Allah
However, ink in tattoos is inserted into the dermis layer of the skin so where the tatoo is located does not prevent water from reaching the skin. Also, some scholars have allowed people to get plastic surgery etc e.g to sort facial features out after an accident or something like that which is changing the creation of Allah too.
Some people already have tattoos and convert to Islam it is a bit unfair for the ink to prevent such a person from praying or for this person to be at blame for changing oneself even though they may not have been aware of the consequences of their actions.