The equity math actually works out better for the multiway pot than a raised pot (assuming 5 way pot first case vs 4bb raise and one caller second). Investing 1bb with ~35% equity in a 5bb pot is much better than investing 4bb with ~52% equity in a 9.5bb pot. It's more bang for your buck in relative terms - .75bb/1bb invested in the first case and .24bb/1bb in the second.
The
equity is higher in a MW pot if you are allin preflop.
However postflop people do not continue in MW pots with nothing, in fact the strength of a hand at SD in a MW pot is usually higher than in a HU pot.
So whilst you are in a favourable position pre-flop, post-flop you will find that if you hit a hand in a MW limped pot with a 'speculative hand' and you get action, you are not in good shape.
People are entering these pots with the intention of giving up if they miss as they are cheap, so hitting your hand rarely leads to a big payoff. However should you hit your hand and get action, then someone else has also hit a hand and whilst you have a big hand you do not have the 'upper end' of big hands because virtually all hands that would have mead that kind of hand would have been raised preflop.
When you raise preflop you alter most of this. You push out many weak hands that hit hands that are hard to spot.
You gain initiative, its a lot easier to push someone off a small made hand with a c-bet than a bet out of nowhere.
Your raise preflop may also take the pot down there and then.
Your winrate is also higher when you raise rather than limp, look at your own database. Filter by pots where you are the PFR vs limpers and compare it to where you call vs limpers. Which has the bigger winrate? Should you therefore be doing more of whatever is actualy winning money rather than trying in vain to reinvent the wheel?