There are, theoretically, unlimited runs available to batsmen (OK, in limited overs games there is a limit, barring no-balls) but only ten wickets available to bowlers. Innings can, and do, occur where three or four batsmen score hundreds. That can't happen with 5-for. 5-for is significantly more difficult most of the time.A century is a century and should be acknowledged for what it is, I have more of an issue with the fact we put a five wicket haul as a comparison to a century when in fact it is clearly a far greater achievement than making a ton.
Taking 5 wickets is half the wickets, rarely does making a century end up being half the runs.
I will say I was a fast bowler and it used to shit me that incentives were paid to batsman making a hundred and fir a bowler only if you got a 5 wicket haul or better.
A lot of A grade pennant clubs did change this to a 4 wicket haul in the early 90's but not sure they stuck with it as I have been out of it for a while now.