In the scene reintroducing Alpha in Once A Ranger, Alpha implies he hadn't been in working order for a while. Who's been working on him to get him up and running? Adam, who had experience with Morphin' era tech, and Andrew Hartford. Who created an Artificial Human. Who better?
The Morphin' Grid is some sort of energy formed from the battle between good and evil, and the run-off energy tends to form in explosions. With multiple bad guys (and later in the team-up episodes, multiple good guys), the Grid must've been overloaded and gone crazy with the explosions.
The seemingly fridge-logical aspect of Andrew making his robot son a young adult in appearance shifts to brilliance because when he finally dies, Mack can live on his own and not be discovered by an adoptive parent.
Andrew creating an artificial son that was physically a young adult, although it's likely that this is a preferable alternative to making new bodies for him to simulate growing up.
If Andrew is skilled enough to create a robot sophisticated enough to perfectly pass for human as much as Mack does, why doesn't he use that technology to create a team of Power Rangers?
He probably would have used up a lot of time and energy for it.
Regarding "Once A Ranger": In the original MMPR series, Rita Repulsa replies on two occasions to Lord Zedd's idea of having children with utter disgust. So, what changed her mind enough to conceive Thrax?
One would expect much alcohol was involved, hence why we never hear of it before now, what with this being a kids show...
So Thrax is the son of Lord Zedd and Rita. Okay, stretch, but we'll go with it. But that makes him only ten years old, tops. And yet he was sealed away by the Sentinel Knight, who was, at the start of the season, incorporeal. So WHEN, exactly, did this happen?
Since Mack is a robot, he won't age like those around him. If Andrew had died before telling him who he really was, Mack would never know why this was happening. Not every day you see a teenager filling up the fridge like this...
Kind of rectified in the end, since Mack found out, and he eventually became human.