Why de-extinction scientists cannot never revived mammoths?

People, who accept the theory of the collision of a big meteorite with earth and other extinct theory, think that by revealing the existing genes in the remained eggs of dinosaurs they can rebuild them, or by using the sperms of frozen Mammoth they can rebuild them. They think that the decline factor is finished, and they can form new giant animals. These scientist are unaware of the earth which they put their foots on it, and they don't as far the gravity of earth is like the gravity this time, it is impossible to revival such giant animals. Mammals can not be larger than the size of an elephant currently. Reducing metabolism rate by increase the body size. The lowest metabolic rate for survival a mammal, is a certain amount and a mammal will die in below amount. It metabolic rate which is in the elephant. A mammoth-sized mammals will have metabolic rate below the limit of life, currently.