MacBook Pros have garbage thermals.
Essentially, the fan at the back is unable to effectively move heat sufficiently to sustain any real workload on a MBP, particularly on older MBPs. All machines — laptops and desktops alike — are natural air purifiers, in that they tend to collect dust out of the air. This isn't good for them, and in particular, a heat vent clogged with dust is less efficient at cooling than one not clogged with dust.
On my personal laptops and desktops, this is a relatively minor issue: open the case, blow some canned air over things to dislodge the dust paying attention to the vents and fans, close the case. I.e., you clean it.
MBPs have several issues that compound here, though: first, they cannot be reasonably opened. Only asshats put torx screws into something, to make sure that it is as painful as possible to disassemble, and to force you into an "Authorized Repair Shop". Apple put pentalobe screws in, which is like next-level asshattery. The only reasonable options were flathead