The decentralization of Ethereum's builder market is a critical issue, as the blockchain's security relies on decentralization. This paper investigates the builder market, which is one of the least decentralized parts of Ethereum. Despite being permissionless, the market has become highly centralized, with three builders producing over 90% of blocks. The study uses a large-scale auction dataset to analyze the reasons behind this centralization and its implications for MEV-Boost auctions. The paper finds that private order flows, which are transactions bypassing the public mempool, are a significant barrier to entry for new builders. Accessing these flows requires a substantial financial investment, which can be prohibitive for new participants. The study also reveals that the true value of bids is often higher than the bid value, indicating that builders can extract more value from blocks than they pay for. This leads to significant inequality in block-building capabilities among builders, with top builders having much greater advantages. The study shows that over 88% of MEV-Boost auctions are competitive, but uncompetitive auctions still result in losses for proposers. Additionally, only about 79% of auctions are efficient, with over half of the inefficient auctions caused by block subsidization. The findings highlight the need for mechanisms to reduce reliance on trust and improve the decentralization of the builder market. The paper contributes to the understanding of MEV mitigation and the economics of MEV-Boost auctions. It provides empirical evidence that can inform future theoretical analyses of these auctions. The study also identifies key providers of private order flows and their impact on the builder market, emphasizing the importance of fair exchange mechanisms to enhance decentralization. Overall, the research underscores the challenges in maintaining a decentralized builder market and the need for improvements to ensure fairness and efficiency.The decentralization of Ethereum's builder market is a critical issue, as the blockchain's security relies on decentralization. This paper investigates the builder market, which is one of the least decentralized parts of Ethereum. Despite being permissionless, the market has become highly centralized, with three builders producing over 90% of blocks. The study uses a large-scale auction dataset to analyze the reasons behind this centralization and its implications for MEV-Boost auctions. The paper finds that private order flows, which are transactions bypassing the public mempool, are a significant barrier to entry for new builders. Accessing these flows requires a substantial financial investment, which can be prohibitive for new participants. The study also reveals that the true value of bids is often higher than the bid value, indicating that builders can extract more value from blocks than they pay for. This leads to significant inequality in block-building capabilities among builders, with top builders having much greater advantages. The study shows that over 88% of MEV-Boost auctions are competitive, but uncompetitive auctions still result in losses for proposers. Additionally, only about 79% of auctions are efficient, with over half of the inefficient auctions caused by block subsidization. The findings highlight the need for mechanisms to reduce reliance on trust and improve the decentralization of the builder market. The paper contributes to the understanding of MEV mitigation and the economics of MEV-Boost auctions. It provides empirical evidence that can inform future theoretical analyses of these auctions. The study also identifies key providers of private order flows and their impact on the builder market, emphasizing the importance of fair exchange mechanisms to enhance decentralization. Overall, the research underscores the challenges in maintaining a decentralized builder market and the need for improvements to ensure fairness and efficiency.