Most of it is due to necessity - they were mostly either created or reintroduced by Marvel during the 60s and 70s when it wasn't unfeasible that they would still be around and they have had to find excuses for it (cryogenics, deaging, immortality, etc) as the years have started to add up.
If you really want to complain, then I suggest bringing up characters like Iron Cross, reintroduced into the present in 2001 and still an active hero - despite having fought in WWI, making him at least 103 when he reappeared, and with no powers or event that could explain it.