6 Reasons your Players can't just Buy Everything

[1] Adversarial. Before the characters are able to make huge or sweeping purchases, the properties or mercenaries are already sold off to someone else first. The person is either a campaign villain, a group of nobles or wealthy merchants that want to limit the player's power, or even a supernatural being such as the God of Trade or the Archdemon of Greed acting as a long term foe for the party.

[2] Cultural. Regardless of how much wealth you have, certain services and privileges are not available unless you prove yourself first. You cannot simply buy a castle, you must reach name level first. The greatest smith in the setting will not forge you a sword of heroes unless you go on a quest or slay a mighty beast, even if you were the richest man alive, and so on.

[3] Currency Tiers. For religious, cultural, or game-abstraction reasons, currency doesn't combine upwards. Paradoxically, you may be able to exchange gold for silver and silver for bronze, but you could never get a single shred of silver for all the bronze in the world and so on. Certain extremely expensive purchases such as buying land from nobility, buying armies, gaining powerful artifacts and so on the payments can only be made in incredibly rare and valuable currencies like platinum or pure gold coins, meaning massive amounts of lesser wealth won't let you buy the most valuable things- this is intentional to keep out the Nouveau riche.

[4] Feudal Obligation. Instead of keeping all the money and riches you find, most are taxed or given to the local lords and leadership, who will instead pass on some of the wealth and honor to the players. As time goes on, they may be rewarded with land or extra privileges instead of keeping the majority of the wealth themselves. In short, you're Skerples.

[5] Realistic Prices. Common in games with a silver standard; your characters may be getting wealthy through dungeons, but not obscenely wealthy. You aren't getting lifetime's of wages every time you go into a dungeon, but you are making enough fluid cash for easy money and luxuries.

[6] Market Fluctuations & Inflation. Once you start selling off manticore skin, the price goes down as supply goes up. You flooding a village with gold coins and loot will mean that everything in that place will go up in price as a result; at least until a local tax authority or traveling noble finds an issue. While this may annoy your players, it also evokes a feeling of actual market change from their actions; maybe don't offload all your wealth on a few dirt farmers next time.