That’s a design decision made and coded into your active theme by the theme’s authors.
As this is a classic default theme (as opposed to a modern Block or Full-Site Editing theme), the only way to override any of these design decisions is through new code of your own.
The following custom CSS code will make the blog post titles on both the archive-type pages (including the blog page you gave) and the individual POST pages bold.
h1.entry-title, h3.entry-title a {
font-weight: bold;
}
(Copy and paste this into APPEARANCE -> CUSTOMIZE -> ADDITIONAL CSS)