Why is the redemption value of a bond usually higher than the initial price of that bond?

Basically, if the redemption value of a bond is higher than the initial price was to purchase it, why wouldn't people just buy an endless amount of bonds and sell them at a markup?

Because your supposition is incorrect. A bond's call price is it's face value.

https://www.investopedia.com/terms/c/callprice.asp

Because the call price (aka redemption value, and relevant only if bonds are called -- paid off early) is fixed at issuance.

OTOH, your question implies that the redemption value is the current market price.

Just as importantly, the company wants you to buy the bonds, and they will only call the bonds if it's advantageous for them to do so.

I understand the redemption value of a bond to be the price an issuer of a bond has to pay in order to redeem a bond before its maturation.

No, the redemption value is the amount that the holder of the bond gets at maturity.

If the bond is callable, the issuer can "buy back" the bond from bond holders at some specified call price, typically the par (redemption) value, but not always. Sometimes bonds have a call value above par (or below). Callable bonds are less common, since callable bonds are less valuable to bond holders (meaning less money to the issuer), and increase risk to the holders.

Issuers also do not call bonds that are trading below the call price, since they could buy them back on the open market for less than the call price (or, more realistically, just forego the call option and keep the bonds until maturity).

I also understand that the owner of the bond can redeem a bond too, which confuses me.

Only if the bond is putable which is even more rare, because it increases the risk to the issuer (and makes bonds more valuable to the holder). Putable bonds do not typically trade very much below their put price for the scenario you outlined (hold the bond and sell it at par).

If you buy a corporate bond with no optionality, you should expect to either 1) hold the bond until maturity, which will yield some gain over the coupon rate if you buy it below par, but it may take many years, so the annual gain is smaller, or 2) expect the bond to increase in value and sell it for a profit, usually because the credit rating of the issuer is bad and you expect it to get better.