Various firms do, both large and small, but it is extremely difficult to compete across so many fronts without a massive amount of capital (logistics, distribution, negotiations with manufacturers, real estate, marketing, product development, etc).

The return on investment for such a massive deployment of capital isn't huge and Walmart could adopt your strategies at any time, undercutting your value proposition. I'm also 100% certain there is a lot more collusion going on but with CEOs smart enough not to write it down in an email.

That said, a tightening labor market is forcing Walmart and other competitors to slowly raise pay.

They have. The reason why Walmart had to raise wages is directly because of this. Walmart was looked upon as the employer of last resort and it was showing in the cleanliness of the stores. The stores were a mess and turnover was huge. Eventually the paymasters had to relent and open the purse strings.

Not entirely true. The wage raising is because of where the market is moving.

However, turnover is actually not as large of a problem as most would like to believe. Cashiers are the positions with the highest turnover rate, many other departments have employees that have been in the same store for many years.

Whys should other companies offer better terms? There is enough unqualified workers everywhere so there is no need to compete for them.