Why are mitochondria absent in bacteria?

Eukaryotes need mitochondria to produce energy (ATP) to function. Bacteria generate energy through glycolysis and making a proton gradient across their cell membranes. Certain bacteria can use light to generate a proton gradient and make ATP that way.

bacteria need to find energy sources from their host to survive!

Prokaryotic cells are less structured than eukaryotic cells. They have no nucleus; instead their genetic material is free-floating within the cell. They also lack the many membrane-bound organelles found in eukaryotic cells. Thus, prokaryotes have no mitochondria

They don't need them.  For those that are aerobic, e.g. like the E. coli in our gut, they have their own electron transport chain embedded in their membrane and it works pretty much just like the one in eukaryotic mitochondria works--except that the protons are pumped into the space between the membrane and the cell wall, and they flow back in to the cytoplasm via their own ATPase, thus generating ATP.  So, in effect, the entire bacterial cell kind of operates like one gigantic mitochondrion.

And FYI, the very origin of mitochondria is now thought to have been from a smaller bacterium that got swallowed up by a larger one, and instead of getting gobbled up it stuck around , generating ATP, and evolved into what is now the mitochondrion.

About 1.5 billion years ago, primitive bacteria took residence inside larger cells, resulting in an intimate relationship that would mold the evolution of more complex, multicellular beings. The bigger cell was eukaryotic, meaning it contained organelles -- structures surrounded by membranes, but the prokaryotic bacterial cell had no such arrangement. The bigger cells feared oxygen, a poison to their existence, but the smaller cells used the oxygen to make energy in the form of the molecule adenosine triphosphate, or ATP. The eukaryotic cell enveloped the bacteria in a predatory fashion, but somehow, the predator did not digest the prey. Predator and prey became mutually dependent. Former Boston University biologist Lynn Margulis cited this endosymbiotic scenario in her theory of the origin of mitochondria, the energy factories of cells, and the reason for their numerous similarities to bacterial cells.

For the same reason you don't have kangaroos in your stomach.  They're too big.